Ten years after he established a set of fair-employment principles for U.S. companies doing business in South Africa, the Rev. Leon H. Sullivan now doubts they will bring an end to apartheid and is hinting that today he will call on firms to abandon them in favor of total disinvestment.

The decision, which has been painful for the 64-year-old civil rights activist, could be costly for companies still doing business in South Africa, since the six-point Sullivan principles provide the moral underpinnings for their operations in that white-ruled nation.

If Sullivan calls for a pullout, the pressure to leave South Africa will intensify for U.S. companies still there. Those that remain, moreover, are likely to face a growing loss of investment dollars as more pension funds, public institutions and universities sell off holdings related to South Africa.

The Sullivan principles, first adopted by 12 companies in 1977, now have been signed by 110 of the 199 U.S. companies still operating in South Africa. More than 100 American companies, though, have sold their South African subsidiaries over the past 18 months.

The Investor Responsibility Research Center lists 18 U.S. companies that have pulled out of South Africa so far this year and has said 15 others are winding down their operations there.

Under pressure from all sides, Sullivan appears to be wavering on his two-year-old warning, repeated frequently, that he would abandon the employment code in favor of the more radical course of economic sanctions if South Africa didn't end apartheid by May 31 -- last Sunday.

Close aides said yesterday that Sullivan, pastor of the Zion Baptist Church in Philadelphia, still hasn't decided what stance he will take in a press conference here today.

"He's going to stake out his new position regarding company involvement in South Africa," said Daniel Parnel of the International Council, the Philadelphia-based coordinating agency for companies following the Sullivan principles.

Parnel said that Sullivan has been agonizing over whether calling for the end to the employment objectives will hurt black workers in South Africa more than it will help the cause of ending the white government's rule over blacks, who form a majority of that country. Corporations subscribing to the Sullivan principles have argued that their black employes would be the real losers of an economic embargo and complete pullout.

"That is the source of his agony," said Parnel.

Sullivan wanted to visit South Africa in May to help him decide what to do, but the government refused to grant him a visa.

Sullivan's searching was evident in recent speeches. Two weeks ago at West Virginia State College, the once all-black school in Institute, W. Va., from which he graduated, Sullivan said he would call for the total withdrawal of U.S. companies and "a total embargo" of South Africa if there was no change by May 31. That statement was a repeat of one made in December.

But Parnel noted that Sullivan left himself some wiggle room in the speech, and pointed out that one reason for the success of his civil rights strategy has been his ability to switch gears when necessary.

Although studies show that many companies that subscribe to the principles are not fully following them, business organizations said they have spent close to $40 million last year on social welfare programs for black employes in South Africa, including education, housing and health. This was twice the annual rate for the past 10 years.

Furthermore, the companies that follow the code have come under attack from a conservative South African research organization for engaging in "corporate civil disobedience" against South Africa's racial separation laws and practices.

This new activism by corporations comes from additions that Sullivan made to the principles in 1984. These additions went beyond his original efforts to desegregate South African work places and improve promotion and training opportunities for black workers. They pressed actual civil disobedience of apartheid laws, including support for blacks who want to attend schools, ride buses and use beaches that bar them.

Many of the companies are preparing to follow the employment principles even if Sullivan renounces them, and some are even looking for a new advocate for that point of view. The companies, banded together as an Industry Support Unit for the principles, call it "doing Sullivan without Sullivan." But the principles likely would lose credibility without the backing of Sullivan.

Nonetheless, Sal G. Marzulla, a Mobil Corp. official speaking as chairman of the Industry Support Unit, said the companies would stick with the Sullivan principles because "we think it is the right thing to do."

But Sullivan has been under increasing pressure to renounce the employment code in favor of strong economic sanctions.

Randall Robinson of TransAfrica, the most activist Washington lobbying group against South Africa, long has urged a greater use of economic pressure against apartheid. He has attacked the Sullivan principles as doing "more harm than good."

"We feel the Sullivan principles have been ineffective and have not produced the results that he has hoped for. We feel that stronger measures are needed," said the Rev. Arthur B. Keyes Jr., executive director of Interfaith Action for Economic Justice.

But Keyes said he believes that Sullivan's decade-long association with corporate America, including his membership on boards, means that he will not come out for complete sanctions against South Africa.