FOR THE FIRST TIME this term, the Supreme Court sat yesterday with a full complement of nine justices. And one announcement made by the court means that the newest member, Justice Anthony Kennedy, has arrived just in time for an extremely important case: the court agreed to decide the constitutionality of the Ethics in Government Act, which authorizes the appointment of special prosecutors in cases involving high government officials. As an indication of the urgency of settling this question, the court will hear the case on an expedited basis -- arguments have been set for April 26 -- and will hand down a decision before the term ends in June.

This prompt action is welcome. The special prosecutor law was passed in 1978, but until recently no investigation had produced an indictment. Now, Michael Deaver and Lyn Nofziger have been convicted in prosecutions brought by special prosecutors, and two minor figures in the Iran-contra scandal have entered guilty pleas. It has been clear for some time that a serious challenge to the law would be made and would have to be resolved by the Supreme Court. Last year, most of the special prosecutors who were then conducting investigations accepted parallel appointments in the Justice Department in order to preserve their work should their authority under the ethics law be invalidated. Two did not. Whitney North Seymour, who prosecuted Michael Deaver, and Alexia Morrison, now investigating three former Justice Department officials -- Theodore Olson, Edward Schmultz and Carol Dinkins -- chose to continue without this backup authority. The case before the Supreme Court involves the Olson inquiry, but the ruling could presumably affect the Deaver conviction as well.

The U.S. Court of Appeals for the District of Columbia has ruled the special prosecutor law unconstitutional, but the question would have gone to the Supreme Court no matter what the finding below had been. The appellate court was divided 2 to 1, and strong opinions were written on each side. Does the ethics law, as the majority found, depart from the constitutional framework of balance of power in authorizing an independent, court-appointed attorney to prosecute crimes? Or, as the judge in dissent and the majority in Congress believe, is it the only practical mechanism available for dealing with wrongdoing at the highest levels of government? There will be a final answer before summer.