Following in what has become a long and spectacularly squalid tradition, yet another president of the Teamsters Union has been indicted on charges of racketeering and corruption. Jackie Presser is the fourth chief executive of the 1.6 million-member union to face federal charges in the last 30 years. A grand jury in Cleveland accused him of participating in a scheme to embezzle more than $700,000 in union funds in order to pay three persons who were on the payroll but did no work. Two other union officials were indicted with Mr. Presser, and, in a related and important action, an FBI agent was indicted here in Washington for aiding the scheme.

The Labor Department's Office of Labor Racketeering had been investigating these charges for three years when the Justice Department's Organized Crime Strike Force in Cleveland presented the facts to the grand jury in that city last July. Then suddenly, and without a full public explanation, the charges were dropped. Mr. Presser, it was rumored, had been in some way cooperating with the FBI on cases involving other union officials, and in return some promises had been made to him. What promises? Who authorized them? Which high officials in the bureau and the department knew about this deal? Why were the Labor Department and the federal prosecutors in Cleveland kept in the dark? And finally, who ever thought up the idea of granting immunity to the whale in order to catch the minnows? Because Mr. Presser had been the only leader of a major union to endorse the president in both his campaigns, and because he was known to have high-up political contacts, many suspected a fix.

The Justice Department, obviously embarrassed by these developments, promised an internal investigation, and it has produced results. Special Agent Robert S. Friedrick, who was until recently supervisor of the FBI's organized crime squad in Cleveland, has been indicted on charges of lying under oath about contacts and agreements he had with the Teamsters president. Two other FBI agents involved in the case are still under investigation and may face charges. The indictments, of course, do not provide details about the bureau's relationship with Mr. Presser, but the evidence presented at trial is sure to produce some answers to the questions raised last summer.

What about the future? Informers are almost always necessary to the successful prosecution of labor racketeers, organized crime figures and major drug dealers. Their identity must be closely guarded, and for this reason, only a few law enforcement officials know all the details of any arrangement. In the case of Mr. Presser -- and who knows how many others, for that matter -- the system was out of the control of responsible supervisors here in Washington. When this internal investigation is completed, the Justice Department and the FBI have an obligation to produce a working plan for preventing any more abuses like these.