ONE OF THE functions of the Federal Bureau of Investigation is to ensure domestic security and protect against terrorism and violence. In a totalitarian society this job is easy. At the mere rumor that dissident groups have formed and are planning to take action of some kind, arrests are made, files confiscated and meetings prohibited. Here the courts and the Constitution safeguard political rights, and national police must operate within a framework that requires them to distinguish between advocacy of unpopular ideas and incitement to unlawful acts. In order to clarify these distinctions and instruct FBI agents on procedures in these sensitive cases, Attorney General Edward Levi in 1976 issued guidelines on domestic security and terrorism investigations.

The guidelines set policy on difficult questions: what kind of activity merits a preliminary inquiry by the bureau? When would it be prudent to start a full- fledged investigation? Under what circumstances should wiretaps be requested, informers used and undercover agents placed? What kind of files should be kept on groups that have not engaged in criminal activity but might do so at some time in the future? How long should an investigation be continued if no criminal activity is being carried out?

The 1976 guidelines have worked well, according to FBI officials, but last week they were amended. All the appropriate language about protecting freedom of speech and limiting intrusive investigative techniques has been retained. No one has charged that the revisions are an assault on constitutional rights. But the American Civil Liberties Union has asked for clarification of three of the changes.

First, it wants the bureau to tighten the definition of criminal advocacy that requires investigation so that it covers only speech that is accompanied by an apparent intent to engage in crime. The distinction is between a statement like, "If we really wanted to show our disapproval of the defense budget, we'd all stop paying our taxes," and one that urges people: "Use my manual to set up a phony church, declare yourself a minister and avoid paying all income taxes." Second, the ACLU suggests that while inquiries can certainly be made on the basis of allegations and tips of uncertain credibility, investigations using undercover agents, wiretaps and surveillance should not be begun unless there is a "reasonable suspicion" of criminal activity. The new guidelines would authorize such full-scale investigations where "compelling circumstances" exist, but that term is not defined. Finally, the revisions allow the bureau to keep files on persons and organizations not under investigation if the material in the files was "publicly available." That term, too, needs a definition.

At least four congressional subcommittees have indicated a desire to hear FBI Director William Webster on these questions. Guidelines in this sensitive area are a good idea, but making a clear record of the drafters' intent can only improve them.