THE HELSINKI AGREEMENT, which was criticized at the time of its signing for seeming to sanction Soviet power in Eastern Europe, is looking better for the United States all the time. Because it included a broad human-rights plank, the agreement soon became a principal basis on which indiviuals within Soviet-bloc nations could demand to be treated by their governments in accord with law. It is true, as was reported the other day by the U.S. commission set up to monitor Helsinki developments, that the Helsinki process had produced real results. In Poland, for instance, a group of protesting workers (and some supporters) had been detained and then were released. If bloc liberalization depends ultimately on its historical transformation, then Helsinki can give history a little push.

But Helsinki has other uses. When 33 European nations plus the United States and Canada met in 1975, Jimmy Carter's election was a remote thing. No one could have foreseen how the Helsinki movement and the Carter crusade would feed into each other. Nor later, when President Carter's initial human-rights bursts against the Russians had provoked cold fury in Moscow, could anyone have foreseen how convenient it would be to have on hand a way to switch over from a one-country assault on Soviet violations to an effort by all 35 signatories to deal with each other's violations. That's what Helsinki has meant: a code of domestic conduct and a continuing diplomatic process of review.

The formal review is to begin on Oct. 4 in Belgrade. Its procedures and agenda have just been hammered out in a seven-week prepatory session in which the United States gained most of what it wanted in terms of time and jurisdictional room to review the human rights record. American diplomats also ensured that there will be a post-Belgrade follow-up meeting - the Russians are not going to be allowed to ditch the subject. In turn, the West accepted Moscow's general approach to the economic and cultural issues of special Soviet interest. The United States did so well because of the political and paychological steam that has built up behind the Helsinki idea, and because most Helsinki signatories are allies or espouse Western values.

The preparatory meeting happened to fall at a time when uncertainty in Soviet-American relations had made it particularly useful to have a domonstration that tough issues could be discussed productively. This was done. The question of how to "play" the human-rights issue will be even more acute in October, when SALT will also be on the Soviet-American plate. But as a result of the Belgrade preparatory meeting, there is now somewhat more reason to hope that Washington and Moscow can prevent their continuing differences from intruding excessively upon their common interests. That's what detente is about.