The Reagan administration has decided to support a $1.1 billion International Monetary Fund loan to South Africa, which would be the largest international loan in South Africa's history. Coincidentally, the $1.1 billion exactly equals the increase in South Africa's military expenditures from 1980 to 1982, during which time it has stepped up its intervention in Angola and Namibia.

One could draw from this the unhappy conclusion that in backing a loan of this dimension to South Africa, this government is indirectly subsidizing South African military adventures in the region.

That's a step far beyond the friendly overtures the Reagan White House has been sending to South Africa -- overtures that so clearly have outraged many Americans who object to that country's rigid racial separation policies.

Early on, Ronald Reagan said: "As long as there's a sincere and honest effort being made to modify their racial policies we should be trying to be helpful." He later paved the path for a visit to America by South Africa's Prime Minister Pieter W. Botha, which blacks and many Americans who believe in justice considered a grievous affront.

During Prime Minister Botha's visit to Washington, I asked him under what circumstances South Africa would be willing to adopt the policy the Africans want of "one man, one vote." His answer came almost in a hiss: "Under no circumstances! Under no circumstances!" So much for sincere and honest efforts to modify racial policies. President Reagan's extension of this ultimate American hospitality -- a visit to the White House -- served notice to close observers in America and Africa that, indeed, South Africa could afford to relax its guard on the racial front.

South Africa applied for the IMF loan in early October, a month after denying reports that it intended to do so. When stories surfaced of South Africa's intentions, 35 congressmen urged Treasury Secretary Donald T. Regan to instruct U.S. representatives at an IMF meeting in Toronto last month to oppose the loan, citing South Africa's "aggressive foreign policies and its internal policies of segregation." In a letter last week to congressmen opposed to the loan, Regan said that to oppose it on political grounds would violate the "apolitical nature" of the IMF.

What "apolitical nature"? The Center for International Policy here points out that "The United States has consistently opposed loans to countries with whom it has political differences," using as a pretext "technical grounds." This includes such countries as Grenada, Nicaragua and the Indochina nations.

Randall Robinson, director of Trans-Africa, a black American lobby for African interests, called the IMF "a good multilateral institutional blind behind which the U.S. can hide its support of South Africa. It is a way of deflecting international criticism of U.S. support."

The fact is, this administration's rule of thumb is that if you're anti-Soviet, you're okay; you can bury your people and this administration will turn a blind eye. South Africa is spectacularly anti-Soviet.

Such policies have contributed to making this administration the meanest and cruelest U.S. administration in the eyes of Third World countries in the post-war era, with the South Africa issue simply the most obvious manifestation. Not only are Americans having to suffer because of the administration's policies, but innocent citizens of other nations are suffering as well.

The U.N. General Assembly has repeatedly called on the IMF not to make any loans to South Africa, but the IMF has declined to comply. Next week, a U.N. committee will hold a hearing on the subject.

Meanwhile, those with the toughest jobs in town are the groups voicing strong objections to policies such as the loan because of their uncompromising stand on South Africa's racial regime, perhaps the most heinously repressive since Nazi Germany. One of these is Robinson, who used to say that he was prepared to go to any length to see that no tilt occurs in U.S. policy toward South Africa at the expense of black Africa. Now that tilt has become policy beyond Robinson's worst fears. Of this latest move, he says, "We are not surprised . . . but we must try to build opposition to it."