Last week I found myself rather bemused to read my friend Alan Keyes' hot-blooded epistle about the collapse of American principle at the United Nations {op-ed, Jan. 21}. My initial reaction was to pass over the matter without comment. After all, the issues are long in the tooth to anyone who follows U.N. affairs, and Alan's insight seems to have come since last we testified in concert before Congress.

But I changed my mind when people began to perceive a substantive difference between Jeane Kirkpatrick and Vernon Walters. From my perspective, nothing could be farther from the truth. Differences in style and tactics are not the same as differences over principle. I could no more be Jeane Kirkpatrick in style than she could be me. But at the risk of sounding self-serving, I would say that to mistake our differences of style for differences in principle is like finding Gen. Omar Bradley less principled because he was not more like Gen. George Patton.

The hand on the tiller of American U.N. policy is President Reagan's. He has instructed me to "reform the place, don't destroy it." I have sought to do just that.

To achieve budgetary reform, for example, we withheld payment of a substantial portion of our U.N. dues. The U.N. depends on the dues of its members to operate. Our agreed share is 25 percent. The pressure worked to begin reform. The size of the U.N. Secretariat staff is now almost 15 percent smaller. For the first time, the two-year U.N. budget is lower in real terms than its predecessor. Reorganization and streamlining measures are in progress.

No one claims that U.N. reform is complete. Reform is not a one-day process; it must go on. But to shut down the U.N. by continuing to withhold our pledged dues is like shutting down the Pentagon until we have defense reform acceptable to all. And as a matter of principle, it is un-American not to pay our debts. Last year Alan Keyes himself advocated before Congress President Reagan's request for full payment.

I cannot hope to explain here each of the complex issues raised in his article, but let me offer a few brief responses to indicate the world is not the simple place he finds it.

Secondment: This is a complicated issue involving the independence of international civil servants. It dates from the League of Nations, and is unlikely to be resolved quickly or neatly. With Alan Keyes' support, we brought the issue back to life in my tenure. I am surprised to see him imply that a few tough words here andthere will solve it. He knows better.

Support for Israel: No one could be more supportive of Israel's right to exist than President Reagan. Our efforts have increased support for Israel in the annual credentials votes. Far from joining "a polemical assault on Israel," we voted as a matter of principle on the issue of treatment and deportation of West Bank residents. This was not a vote against Israel. Although the United States has voted against Israel in the past, I have never done so. Friends, as the United States and Israel are, can differ on individual issues.

We are also cited as failures for the withdrawal of a U.S.-initiated resolution tying self-determination to free elections. Alan Keyes is well aware that we forced the Third Committee to confront in public debate the hypocrisy of separating the two concepts. We then withdrew our resolution because of an amendment that would have been interpreted as an attack on an ally -- Israel.

U.N. Voting Records: Keyes cites as our "only success" the absence of anti-U.S. name-calling resolutions. This is a misleading statement. Hostile resolutions that never see the light of day are victories for us. When we compile our voting statistics, however, there is no way to count such things. Over 55 percent of U.N. resolutions are decided by consensus, meaning that we have stuck to our principles and others have come more or less to see things our way. When we change "No" votes to abstentions, we are moving world opinion our way. We have seen this happen repeatedly. Each year since my arrival, furthermore, we have achieved larger majorities condemning Soviet actions in Afghanistan and Vietnamese actions in Cambodia.

I see our role today at the U.S. Mission to the U.N. as one of building upon the superb foundation laid by Jeane Kirkpatrick under the direction of President Reagan. Because of the president's steadfastness, U.S. interests are today better protected in the U.N. than they were seven years ago, despite the crippling handicap of massive withholding of U.S. dues.

I have learned, since first I interpreted for President Roosevelt almost five decades ago, that American interests in an often unfriendly world are usually advanced by daily painstaking attention to the details of the individual battle while keeping in mind the global perspective. Let me assure all that the sky is not falling in Turtle Bay. What I would like to suggest is that principle is more than posture, and statecraft is more than a periodic public tantrum.

The writer is U.S. ambassador to the United Nations.