This Friday marks the 25th anniversary of an important milestone in diplomatic history that is both significant and sad.

Significant because it demonstrates that the United States and Russia can hammer out workable agreements when the national interests of the two super-powers happen to coincide.Sad because it also demonstrates that such breakthroughs don't necessarily generate more of their kind.

The anniversary involved marks the signing on Aug. 5, 1963 of the limited test ban treaty by which the U.S., Russia, and Britain banned nuclear testing in the atmosphere, under water, and in outer space.

If it weren't for this treaty, the world would have a much more serious form of pollution to worry about than just smoke and automotive emissions in the air and garbage dumped in the water.

But the treaty, seen at the time as only a small first step toward the elimination of the nuclear threat, did not turn out to be the healthy vine from which bigger and more important fruit would grow.

In fact, nuclear testing simply went underground, with the result that the annual number of test explosions has increased in the years following the limited test ban treaty.

This experience should be kept firmly in mind as the U.S. and Russia strive to negotiate a comprehensive nuclear control agreement covering long-range missiles. It should be clear by now that such an agreement won't necessarily grow out of the recently-ratified Intermediate Nuclear Force pact by which Moscow and Washington are to eliminate their short- and medium-range missiles.

Though the super-powers had hoped to sign a Strategic Arms Reduction Treaty (START) before President Reagan leaves office next January, that timetable is now out of the question.

Yes, the two nations have made big strides toward a START pact, agreeing that they would roughly halve their arsenals of long-range nuclear missiles and bombers to 6,000 warheads and 1,600 launchers apiece. But the negotiations have bogged down over the details, in particular how to limit sea-launched cruise missiles - slow-flying nuclear-tipped drones that can be fired from submarines or surface ships.

Other problems include how to keep tabs on the new generation of mobile land-based missiles, and on Mr. Reagan's Star Wars project for a high-tech anti-missile system, whose development Moscow wants to thwart.

After 25 years of experience with the nuclear test ban pact, mankind should have learned that treaties alone are no substitute for the basic changes in the human mind and heart that are needed to make the world safer. But mankind should also have learned that the slow, limited progress represented by nuclear agreements is better than no advances at all.