Refugees from Laos call it ''yellow rain.'' The US State Department has another way of describing these powders dropped from planes. It says Soviet-backed forces in Laos, Cambodia, and possibly Afghanistan have used deadly biotoxins produced by the live fungus fusarium.

Since September American officials, led by Secretary of State Alexander M. Haig Jr., have charged that Soviet-backed forces are using a new kind of biological weapon, in addition to more conventional chemical weapons.

The government charges parallel in more qualified fashion the allegations made by journalist Sterling Seagrave in his recent book ''Yellow Rain.''

These biological warfare accusations have produced serious controversy.

One critic of the government's handling of the charges, Harvard University biochemist Matthew Meselson, says the administration has ''preliminary evidence with a good deal of room for reasonable doubt.'' Too few outside scientists were initially consulted for advice, Dr. Meselson maintains.

Attention has focused on the allegations of biological warfare. One result is that the administration's far-stronger evidence concerning conventional chemical weapons (which may be just as lethal for civilians) is largely ignored.

''One of the unfortunate consequences of the current debate,'' a US congressional source says, ''is that the plight of the victims is largely forgotten. The issue has been distorted into a technical political argument over exactly what kind of weapon is being used.''

The charge of biological warfare has grabbed the spotlight because the evidence is controversial and because the political implications are more far-reaching than in the case of chemical warfare.

If the US government can make its biological warfare charges stick, the Reagan administration could gain increased public support for major changes it has long favored in US policy toward the Soviet Union.

Here are some of the major ramifications of the the administration's allegations:

* If the Soviet Union has supplied these particicular biotoxins to allies, Moscow would appear to be in violation of a 1972 pact banning production, storage, and transfer of biological weapons, including biotoxins. But if the weapon is merely chemical, such as tear gas or even lethal nerve gas, the Soviets may not be in violation of the 1925 Geneva Convention against chemical warfare. The reason is that the pact has sometimes been interpreted to apply only to state-to-state warfare, and not to internal conflicts.

* If the Soviet Union can be shown to be breaking an international agreement by developing and testing a ''new generation'' of biological weapons, the Reagan administration's advocacy of stricter inspection for all future arms-control agreements could gather wider public support. The victims of possible chemical or biological attack have thus become pawns in the arms race between the US and the Soviet Union.

* Evidence of Soviet biological warfare use could open the way to more funds for US chemical warfare preparedness. Some advocates of chemical warfare suggest the US has allowed the Soviet Union to gain too great a lead, especially in Europe. Although this viewpoint is not universally held, there are those in the US military who want a vastly stepped-up program. (In 1969 President Nixon forswore the first use of chemical warfare and renounced all but defensive preparations against biological warfare.)

The State Department's biological warfare charges rest on three legs of controversial evidence.

One leg is the US laboratory analysis of leaf and stem samples brought from Cambodia and Laos. Some of these analyses found up to 20 times the level of the mycotoxin (toxin produced by living fungus) on the stem as would be expected to naturally occur.

Another leg of the evidence concerns the victims' symptoms. State Department specialists say that bleeding from the mouth and elsewhere is typical of mycotoxin poisoning.

One more leg of the evidence is the government contention that these mycotoxins are produced in cool, not tropical, climates, and require sophisticated large-scale laboratories for production. By implication this points the finger at the Soviet Union, since governments of Cambodia, Laos, Vietnam, and Afghanistan presumably lack the means to produce mycotoxins in hot, sometimes tropical climates.

Also, the Soviets have had wide experience with mycotoxins. In a World War II outbreak in the Ukraine, mycotoxins accidentally produced on grain stored during winter in damp conditions caused some 60,000 civilian casualties.

All three contentions have been challenged by some scientists. They have questioned the scientific reliability of verification procedures for the contaminated stems. They ask, could these have been accidentally or purposely contaminated by some political group seeking a positive conclusion - or could the mycotoxin have resulted naturally?Another controversy has swirled over whether the mycotoxin symptoms have been widely observed. Indeed, only a limited number of Lao refugees actually have reported the symptoms - a factor the government explains by suggesting that many casualties have been caused by chemicals other than mycotoxins. Harvard's Dr. Meselson stresses a need for improved procedures to ensure the samples are authentic. He calls for safeguards to demonstrate that any mycotoxin presence is not explainable by natural or nonmilitary factors. There is also uncertainty whether the mycotoxins quickly produce the symptoms in question, he suggests.The government sometimes seems to exaggerate the prevalence of symptoms to support its conclusions. For example, one State Department official maintained to this correspondent that he would provide documentation showing widespread mycotoxin symptoms among poison victims in Afghanistan. But the material he presented showed scarcely anything related to the specific mycotoxin symptoms.One expert on mycotoxins, US Agricultural Research Service microbiologist Alex Ciegler, who argues that the government may be right in concluding mycotoxins are behind the symptoms, is still skeptical of the contention that large sophisticated laboratories are needed for mycotoxin production.''All you need is the fungus, a few flasks, and some rice, or corn grits. You could produce it in an ordinary kitchen,'' he says.Moreover, as one longtime American expert on chemical warfare has observed, such charges are just about impossible to prove or disprove completely - short of unlikely feats such as actually capturing a biological weapon intact or being allowed to visit a battle area by a government under accusation.This is illustrated by the results of a four-member UN team that recently concluded that it could neither prove nor disprove the US charges. It said a major reason was that it was unable to conduct on-site inspections in Laos, Cambodia, and Afghanistan.Despite the current furor over the biological warfare accusations, charges that chemical (as opposed to biological) weapons have been used by Soviet-backed forces go back a long way. Governments of Laos and Vietnam have denied the allegations.Since 1976 officials of the US and other countries, journalists, doctors, and others have been investigating refugee accounts of the use of chemical weapons in Laos. Many of these reports have come from Hmong hills people, often members of the Vang Pao army, which was formerly backed by the American CIA. Thousands of these have been driven from Laos after resisting government efforts to resettle them at lower altitudes. After 1979 similar refugee accounts began to emerge from western Cambodia, where the Soviet- and Vietnam-backed government of Heng Samrin is battling anti-Vietnamese Khmer Rouge guerrillas armed by China.Intermittent reports also began to suggest Soviet or Soviet-backed Afghan forces have begun to use chemicals on Afghan insurgents seeking to reverse the late 1979 Soviet invasion of their country.In many cases although refugee accounts were compelling, they were not strong enough to meet exacting scientific standards.In Laos, for example, the reports often came from refugees affiliated with antigovernment groups. If, in fact, chemicals were being used, were they used in a limited way against pinpointed government opponents - or were they used in a campaign of ''genocide'' against the Hmong people as a whole? Was whatever was being used a startling new weapon under testing, or was it some familiar, generally nonlethal but sometimes fatal agent such as riot-control gas or even phosphorus?US diplomats and aid workers based in Laos could find little evidence within the country suggesting chemical attacks. Despite the movement of people into the Lao capital of Vientiane, foreigners heard no accounts from Laos themselves that confirmed large-scale chemical attacks in the countryside. But those who interviewed refugees in Thailand often found their accounts extremely compelling.Persuasive charges about chemical warfare from Cambodian refugees have also demanded caution because it was obvious that the Khmer Rouge was using these charges as a propaganda weapon against Vietnam. From Afghanistan reports were sketchy and relatively infrequent, often suggesting that the more generally used weapon was nonlethal incapacitating chemicals such as riot-control or tear gas.Difficulties in getting precise impartial evidence have thus led to a debate sometimes centered on abstract, technical, and theoretical questions about how much circumstantial evidence is enough to justify strong US accusations.Believers in the administration charges have criticized doubters whom they say refuse to accept facts even when face to face with ''the smoking gun.'' But the doubters, who often urge more scientific rigor, have argued that in this case there is more smoke than fire.Still, many who have gone through the mounds of refugee testimony, conflicting opinions, questions, and political biases have concluded that where there is enough smoke, there must certainly be at least some fire.

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