In a nine-page booklet entitled ''The Unseen Conflict - Foreign Espionage Operations Against the United States,'' the Federal Bureau of Investigation offers advice about foreign spies to employees of high-technology firms:
''Spies do exist, and several hundred spies . . . now operate within the United States. . . . The bulk of these spies has been dispatched by the Soviet Union, but the USSR's allied nations in Eastern Europe, as well as Cuba and other nations, also operate . . . within the US. . . .
''Increasingly, advanced US technology - much of which is barred from export to the Soviet Union and Soviet-bloc countries - has become a major intelligence target. . . .
''The operative of a foreign intelligence service need not be a foreigner, nor need the occasion of your encountering him be . . . extraordinary. A routine acquaintance, for example, could turn out to be a diplomat from Eastern Europe or an American who has been recruited as an agent. . . .
''Do not expect either the intelligence officer or agent to expose his role in any dramatic or sudden fashion. Usually there is a long period of cultivation. . . . At any point where someone begins to inquire aggressively into aspects of your knowledge or activity which are classified or . . . sensitive, you should certainly stop to consider whether the inquiry is normal, innocent curiosity. It might be . . . an attempt to secure intelligence information for . . . another country. . . .
''Casual meetings away from the office should be avoided. Gifts or special favors should not be solicited or accepted. One further defensive technique is to avoid one-on-one meetings. Needless to say, the more people involved in a meeting, the less opportunity there is . . . to develop a personal rapport or to ask the employee questions he does not want to answer.''