MARVIN GREEN was 23 years old when he was arrested in Kansas on federal charges of conspiracy to distribute drugs.
Police found 5 grams of crack cocaine in a car he was riding in. The driver escaped, leaving Mr. Green facing up to 40 years in prison, even though he had never been charged with anything more serious than failure to pay a traffic ticket.
But with his family's help, Green hired a good lawyer, rejected a deal that would have sent him to prison for years, and was acquitted. ''I was going to get the 40 years - that's what everyone was telling me,'' Green recounts. ''I was the first black person to get acquitted in those federal courts in Kansas.''
The charge that the criminal justice system treats blacks harsher than whites is not new. But at a time when states are passing tough sentencing laws and spending millions of dollars to build more prisons, there is fresh evidence - and a Supreme Court case - that may force a hard look at the system's equity.
Recent studies show that an extraordinarily high percentage of young black men are under the supervision of the criminal justice system. Nationally, 1 out of 3 black men in their 20s are in jail, on probation, or on parole. A study just released here reports that in California the proportion is even higher, 4 out of every 10.
The studies' authors charge that the high rate is a function of racial discrimination, driven in large part by the ''war on drugs,'' which has fallen disproportionately on minority communities. The US Supreme Court this week heard arguments in a case claiming federal prosecutors chose to try blacks in federal courts, rather than state ones, because federal courts mete out stiffer sentences in drug-trafficking cases.
''When Latin Americans and African-Americans come under the power of the criminal justice system, at every step of the way the dominant society - whites - looks at them differently,'' says Vincent Schiraldi, author of a recent study on how blacks are treated in California.
Five percent of young white men are in prison, the study shows, compared with 11 percent of Hispanic men, and 40 percent of young black men. It points to national evidence showing, for example, that while blacks and whites use illegal drugs at about the same rate, blacks are arrested for drug offenses nearly five times as often as white youths.
Critics of the studies retort that the higher rate of arrests and convictions accurately reflects the level of crime in the black community. ''I do not believe race is the primary factor in those numbers,'' says Kent Scheidegger, of the Criminal Justice Legal Foundation in Sacramento, a conservative research group. ''The very high and very regrettable rate [of blacks] under the criminal justice system is due to a very high and very regrettable rate of crimes committed.''
Some criminologists point to the nationwide crackdown on the sale of illicit drugs as the principal cause of the high proportion of black men in the criminal justice system. ''The drug war has become a race war,'' says Joseph McNamara, a retired police chief and research fellow at Stanford University's Hoover Institution.
Mr. McNamara argues that the higher rate of arrests simply reflects the concentration of policing in inner-city areas. ''Most of the effort - the war - is in the poor neighborhoods where street retail stuff is going on,'' he says, rather than in suburban areas. And the vast majority of arrests are for possession of drugs, rather than trafficking - some 1 million out of 1.3 million drug arrests nationally, he says.
Criminologists have pointed to a racial bias in the sentencing for different drugs - most noticeably the vastly higher sentences in federal law given to users of crack cocaine, the drug of choice in the black community, over the powder cocaine favored by suburban whites. Possession of 5 grams of crack is treated the same as possession of 100 times more powder cocaine, for example.
''If I sell an ounce of crack cocaine and you get caught with four ounces of powder, I'm going to the pen and you're going to a drug-treatment program,'' says Willie Windom, a former convicted East Palo Alto drug dealer who is now a drug counselor.
THE disparity is particularly large on a federal level. But last fall Congress rejected a US Sentencing Commission suggestion to equalize punishments.
In the case argued before the Supreme Court this week, five Los Angeles defendants charged in 1992 with operating a crack house are claiming that prosecutors steer blacks into federal court while whites are tried in state court, where sentences are less severe. Public defender Barbara O'Conner submitted a study showing that all 24 of the federal crack cases in 1991 in Los Angeles involved black defendants.
The case tests what evidence is sufficient to support a claim of selective prosecution, thus requiring prosecutors to disclose why they chose federal or state courts when prosecuting individuals.