House-Senate conferees considering the Export Administration Act have agreed to one of the strongest anti-apartheid measures ever taken against South Africa.

The action, taken late Monday night, would prohibit loans by American banks to the South African government "or any of its entities" -- far less than opponents of the administration's policy of "constructive engagement" had hoped for but still a significant reduction of U.S. support for South Africa's official racism.

According to Rep. Stephen J. Solarz (D-N.Y.), sponsor of the measure, the amendment covers roughly half of the total economy of South Africa, which now gets some $400 million a year in loans from American banks.

In addition to the prohibition on bank loans, the measure also requires anual reports from the State Department -- similar to the human-rights reports already in effect -- on the degree to which the American firms doing business in South Africa are complying with a tough set of fair-employment principles.

Conferees were not expected to complete work on the Export Administration Act until late in the week, but the South African sanctions appear to be locked in place. According to Solarz, passage of the conference bill is almost certain in the House and "likely" in the Senate.

Opponents of apartheid had hoped to gain passage of legislation introduced by Solarz and Rep. William H. Gray (D-Pa.) that would bar further importation here of South African krugerrands, place civil and criminal sanctions on American companies that violate the fair- employment principles and prohibit new U.S. investments in South Africa.

Still, Solarz was happy. "It's the first time we've ever applied economic sanctions to South Africa," he said yesterday. "This action repudiates the whole policy of 'constructive engagement.' It represents our judgment that we have a better chance (of affecting apartheid) through the application of pressure.

"It's unfortunate that the Senate was unwilling to accept the Gray prohibition on new investment, the most significant piece of the whole package, but Bill Gray's amendment was very helpful in creating the climate for what we did achieve."

It's quite an achievement at that. The administration has been made to look silly in its attempt to use the carrot of "constructive engagement" instead of the stick of sanctions to move South Africa toward accommodation on a number of issues, including some significant softening of apartheid and a settlement in Namibia. South Africa, apparently reading the U.S. position not as enlightened diplomacy but as weakness of will, has dallied for almost the whole of the Reagan term.

Assuming final passage of the measure (with time running outuntil adjournment at the end of this week), it will be interesting to see whether President Reagan dares to veto the bill.

Solarz thinks he won't. "Even a conservative administration would have second thoughts about vetoing it because of this," he said. "And if he does, it would give the whole issue visibility of the sort that could hurt him in the presidential campaign."

While Solarz would have been happier with "the full loaf" of civil and criminal sanctions against American businesses that violate the fair-employment principles, he thinks the "half-loaf" adopted this week might serve nicely. "Even without the sanctions, public scrutiny (as a result of the annual reporting requirement) should generate sufficient public pressure to force compliance.

"If anybody at the beginning of this session of Congress had thought it possible to get even this much through, he would have been thought wildly unrealistic."

True enough. And if the most militant American foes of apartheid won't be exactly thrilled with this week's success, Solarz is convinced that "there will be weeping in South Africa."