JOHANNESBURG, JUNE 9 -- Mangosuthu Gatsha Buthelezi, the moderate Zulu leader, today appealed to the Rev. Leon H. Sullivan, the American author of a code of social responsibility for U.S. firms in South Africa, to remain involved in the struggle against apartheid.

Buthelezi told Sullivan that if he made good on his decision last month to abandon his involvement in South Africa and press for a complete withdrawal of American investment here, he would deal nonviolent change a serious blow and force the government of President Pieter W. Botha into an even more intractable position.

"There is now more urgent reason than there ever has been for your continued involvement in the South African situation," Buthelezi said in a letter to the Philadelphia Baptist minister.

Sullivan's 1977 fair-employment code of conduct was designed to encourage U.S. companies operating here to change their policies to the benefit of blacks.

Following his announced withdrawal from the antiapartheid campaign, it was disclosed that Sullivan had been denied a visa to visit South Africa, where he had intended to announce his call for a complete withdrawal of American firms.

Buthelezi, who is widely regarded as an essential participant in any negotiations between the white minority government and the black majority for an end to political and social conflict, implored Sullivan to reconsider his decision.

Asserting that "time is running against" Botha, Buthelezi said, "Be with us when history shakes Mr. P. W. Botha and drops him in ignominy because he has failed to meet the challenge he in fact set out to meet."