President Reagan has now responded to Mikhail Gorbachev's sweeping disarmament proposal of Jan. 15. He sent his response on the eve of the every-five-years Communist Party congress, which opened in Moscow yesterday. Mr. Gorbachev greeted it with a complaint that Mr. Reagan, while agreeing in general terms with some of his proposals, was adding unacceptable conditions.

To the lead item in Mr. Gorbachev's proposal, a catchy call to eliminate all nuclear weapons by the year 2000, President Reagan had a dull answer. He said it was necessary also to establish a conventional-force balance, to ensure Soviet compliance, to ease regional conflicts and the like. And so it is. Mr. Gorbachev has not made the case for a nonnuclear world, let alone offered a practical way to get there.

Mr. Gorbachev had made the fencing-off of the Strategic Defense Initiative a condition for deep cuts in offensive arms. Mr. Reagan resists negotiating a fence. Who expected otherwise at this stage?

Mr. Gorbachev had detached from SDI a couple of other issues. One of them is to extend the existing ban on nuclear testing to cover underground tests. His offer of on-site inspection visibly diminishes the force of one administration argument against a full ban. But, for another reason, President Reagan is still opposed to it: he wants to test, for SDI and other programs. A ban would not of itself make deterrence safer or arsenals smaller -- the principal American considerations. It would, however, impose a qualitative arms cap, help the great powers enforce nuclear nonproliferation and spare the administration a bruising in world opinion. Mr. Gorbachev has the option to keep pressing for it.

The administration had identified Mr. Gorbachev's proposal to eliminate medium-range missiles in Europe as the closest to agreement. Certainly his move to a variation of the Reagan "zero option" and his detachment of this issue from SDI appear to open a way. The Soviet leader asks, however, that the Americans deliver Britain and France to a nuclear freeze. Mr. Reagan could not have failed to indicate that this constitutes an intrusion on sovereignty unthinkable in an alliance of democratic states. Meanwhile, he has made his own proposal, for phasing out the medium-range missiles in three years rather than Mr. Gorbachev's five to eight.

Mr. Gorbachev broods that there will be no second summit if the United States does not shape up on a test ban or a deal in Europe. He is under pressure to show the assembled comrades not so much that his opening to Washington will pay off, but that he won't be pushed around. But surely he knew when he started that great-power negotiation is a tortuous game. Soviet proposals are receiving a fair hearing and a full political massage in the West. The Kremlin should be prepared to weigh Mr. Reagan's proposals no less thoroughly. Negotiation goes on.