NOTHING BETTER illustrates one particular change Jimmy Carter has wrought in Soviet-American relations than the American government's release of two convicted Soviet spies in exchange for the Kremlin's release of five "prisoners of conscience." Spy swaps, of course, are old hat. But this latest deal is something new. It seems to be the first exchange of one sort of Soviet citizen for another sort -- of spies, valued on the Soviet side, for dissidents, valued on the American side. It is barter in Carter coin.

The arrangement became possible only because President Carter made the dissidents as a group valuable by investing so much of his personal and official prestige in them. He made them the political equivalent of spies. Once he had created this new value, it remained only for the Soviets to bargain out a package. This they did, by the way, in a manner akin to the way the Democratic Party puts together a ticket in New York. In their various aspects, the five released dissidents represent Russians and Ukrainians, civil and religious dissidents, Russian Orthodox, Jews and Baptists: something for almost everybody.

Was it a fair exchange? Unquestionably so. Doubtless there are some American officials with reservations about the swapping of captive spies, who are usually in short supply, for captive dissidents, who are usually more plentiful. The precedent tells the KGB that its agents, once caught, may not have to languish long in the coils of American law. Yet the precedent also tells the KGB that its uncaught agents -- those whom American authorities know about but have not yet lowered the boom on -- are subject to apprehension in the United States precisely to be traded for jailed dissidents. It is bound to make for some interesting discussions between the foreign intelligence bureau of the KGB and its bureau for domestic control.

Almost certainly it is a precedent that will please Soviet dissidents. For it is very hard to argus that Moscow would arrest a prominent dissident, with all the obloquy that now brings down upon the Kremlin, just to get trading bait. Dissidents are going to be in hot water, but they would be in hot water anyway. Most Americans are likely to be more eager to rescue a prisoner of conscience than they are reluctant to release a spy. It is a strange mixture of apples and oranges, but it is one condoned by the Western moral code and American public opinion as well.

Soviet dissidents now know that, at least in certain extraordinary circumstances, official American influence extends into the very gulag -- the term for the Soviet prison system burned into Western consciousness by Alexander Solzhenitsyn. Anatoly Scharansky, a dissident for whom President Carter has repeatedly expressed personal concern, remains in a prison camp. But the possibility of American rescue is no less stunning for being available only for selective application. A new element has been added to the human-rights equation.

Typically, the noble Andrei Sakharov, a dissident still living in Moscow, at once called international attention to Soviet human-rights figures who remain in prison. Only a general amnesty of Soviet political prisoners, he said, could provide a real basis for confidence between the United States and the Soviet Union. He is right to remind us all that the system that generates and holds political prisoners is intact. It was not for humanitarian considerations but for political ones -- quite possibly to sweeten the atmosphere for SALT and for expanded trade -- that the Kremlin acted.Yet these political interests are congruent with American interests, and one must respect the Soviet leadership for taking this step, which is not without controversial aspects from its point of view.