AN UNUSUAL number of stories in the past few days have focused on the federal government's use of informers in the course of criminal investigations. These cases do not involve innocent citizens who come forward with information out of a sense of civic duty. Nor do they concern government undercover agents who infiltrate an operation to gather evidence. We are talking about the kind of informer who is himself an unsavory character, often originally part of the criminal conspiracy being investigated, who offers information in exchange for some kind of special treatment by the government.

The Jackie Presser case apparently involves this last element. After a 32-month investigation of the Teamsters president, federal prosecutors in Cleveland have abandoned their plans to prosecute Mr. Presser. It has been reported that the prosecutors' decision was based on the fact that Mr. Presser had been an FBI informer -- an allegation he denies -- and that the Justice Department had not been told of the full extent of his involvement in current investigations. In another case, that against former secretary of labor Raymond Donovan, the Justice Department is looking into charges that an informer, Michael Orlando, has been in touch with the defense as well as the prosecution, a development that may jeopardize this important case. And finally, we have learned in recent weeks that for many years before 1975, the government followed a policy of not prosecuting American spies but using them, instead, as double agents. They became, in effect, the same kind of informers as those the FBI uses in other domestic cases: protected and continuing to cooperate with criminals, because the information they provided was so important to the government. The practice, in the spy cases, may have severely misled the public about the extent of our espionage problem and may have lulled other potential spies into believing that their crime is seldom detected.

If there are so many problems with informers, why are they used at all? The answer is simple. Without them there would be very few successful prosecutions in organized-crime, drug-dealing and corruption cases. The Justice Department has strict guidelines for using informers -- first published in 1976 and strengthened and revised in 1980. The guidelines set out in detail the kind of activities by informers that will be tolerated in order to gain inside information. Participation in criminal activity will be allowed to some extent in order to protect an informer's cover, but initiation of that activity is forbidden. So is any act involving violence. But the FBI, using these guidelines, is responsible for the day-to-day supervision of the informer and his identity and activities must be closely held information. The Justice Department keeps no list of current informers and does not authorize specific contacts and arrangements. Sometimes, therefore, trouble arises when a case moves from investigation to prosecution, as it did in the Presser case.

The Senate Permanent Subcommittee on Investigations has announced an investigation of the handling of the Presser case. It would be a good idea if the senators looked not only at the Teamster case, but at the Justice Department guidelines. Are they sufficient? Are they being observed? Is there a better way for the FBI and the Justice Department to cooperate during an investigation so that neither the prosecution of an important case nor the life of an informer will be jeopardized? These broad and important policy questions should be explored.