IF the United Nations Educational, Scientific, and Cultural Organization was ever going to have a future of substance, its longtime director general, Amadou Mahtar M'Bow, had to step down. The Senegalese educator had run the organization in the autocratic manner of an African chieftain. The agency had acquired a bitter anti-Western slant, which clouded its worthy purpose; it was also badly mismanaged and financially strapped. Mr. M'Bow's decision last weekend not to run for a third term was made reluctantly, under strong pressure from Moscow and the West. But the organization now has a new lease on life; members can rethink UNESCO's mission in a less politicized atmosphere.
The United States left UNESCO in 1984, taking care not to include M'Bow in its list of complaints; Britain and Singapore withdrew in 1985. None of the three are likely to return quickly. Needed reforms, already under way, must be continued. But M'Bow's stepdown makes threatened exits by Japan, West Germany, and others less likely.
Selection of Spanish biochemist Federico Mayor as the new director general should help steer the agency in a more constructive direction. Mr. Mayor pledges correctly to continue reforms and work for the return of former members. He wisely puts the needs of developing nations, particularly those in Africa who were among M'Bow's strongest supporters, at the top of his list.
Over the years, UNESCO has contributed quietly but usefully to the fight against illiteracy and to increased cooperation among scientists in pinpointing the future needs of the planet. Another agency goal - one of improving communications facilities in the third world - got off track for a time in informal proposals to restrict and license Western journalists. The aim remains valid.
With a new director general, the door will once again be open to serious discussion of UNESCO's priorities and agenda.