The House yesterday voted to prohibit the Export-Import Bank from supporting private business transactions with South African purchasers unless the secretary of state certifies the purchaser has endorsed and proceeded to implement prinicples of nonsegregation and equal employment.

While it was the first time Congress has legislated on Export-Import Bank supported trade with South Africa, the language adopted was weaker than that adopted by the House Banking and Currency Committee.

The Banking Committee had voted to forbid the bank form supporting any private business transaction with South Africa unless President Carter determined there was porgress toward the elimination of apartheid.

Rep. Paul Tsongas (D-Mass.), author of the Banking Committee version, said he had agreed to go along with the weaker version offered by Rep. Tom Evans (R-Del.) because "we counted the votes and we didn't have them."

"We were in danger of losing everthing" if they stuck with his stronger ban, Tsongas said.

Evan's amendment was adopted by voice vote, but an amendment by Rep. John Ashbrook (R-Ohio) to knock out any language on trade with South Africa was defeated, by 219 to 116 vote.

Tsongas said the effect of the amendment would be largely "symbolic" and would probably not have any real economic impact, but he called it an "important step" in indicating to the government of South African racial policies and was "willing to pay the price in economic fallout."

Tsongas' original amendment would have been a devastating blow to the ability of South Africa to purchase goods and services from the United States.

Executive branch limits have already been imposed on the Ex-Im Bank regarding South African loans and credits, but the bank still guarantees some $204 million in support of U.S. exports.

Ex-Im Bank speaks to boost U.S. foreign trade by providing for overseas sales by Amercian Arms. The House is considering a bill to increase the lending power of the bank from the lending power of the bank from $25 billion to $40 billion through fiscal 1983. The increase is part of an administration drive to reverse the adversebalance of U.S. trade by increasing U.S. exports.

However the administration did not seek the amendments on trade with South Africa and opposes restrictions on trade with that country.

The issue is sensitive because present U.S. policy has come under increasing fire by human rights advocates who feel the United States should be doing more to show it is serious about ending apartheid in South Africa.

The State Department neither opposes nor encourages trade with South Africa.

While the House language was watered down, it is still stronger than language adopted recently by the Senate Banking Committee which avoided singling out countries for trade restrictions but allows the president submit a list of countries which are eligible for export-import financing.

The efficacy of economic sanctions against South Africa is a subject of debate both by civil rights advocates in that country and here. Some civil rights advocates feel it would only hurt blacks, who would be the ones to suffer the unemployment resulting from it. Others feel it is necessary despite the economic cost.

Black Caucus chairman Parren Mitchell (D-Md.) said yesterday the amendment adopted was "well-intentioned" but "meaningless" in the light of laws in South Africa which mandate discrimination and set jail terms for violations.

Nothing the amendment was a compromise, he said, "You can't compromise with total evil. You can't take a mid-point on immorality."

The House also voted, 179 to 133, to strike out of the bill a committee voted exemption for China from the bank's prohibition against allowing transactions with communist countries without presidential approval.

Supporters of the amendment said it would not effect current trade or extend credits, but was a "symbolic" gesture of frienship with China.

Opponents argued that power to exempt communist countries should remain with the president and that it would be hypocritical to extend an exemption to China, whose human rights policies are as repressive as South Africa's while putting in restrictions on South Africa.

The House is expected to finish consideration of the bill next week.