Citicorp yesterday denied that it was pulling out of South Africa in response to increased pressure from states and cities, anti-apartheid groups or public protests, but experts predicted that more companies would follow the bank's action in coming weeks.

The bank, the nation's largest, is the 136th U.S. company since 1984 to leave or declare its intention to leave South Africa, and it was the last American bank with direct operations in that country.

Citicorp also was the second firm to announce a withdrawal since the Rev. Leon H. Sullivan called on business to pull out unilaterally two weeks ago.

Sullivan is the former sponsor of the code of conduct under which many U.S. companies had operated in South Africa. Ford Motor Co. said over the weekend it is negotiating to donate its South African auto operations to a trust that would be held by employes.

"You will have much more of this. Many more companies will be withdrawing from South Africa in coming months," Sullivan said in a telephone interview yesterday. "Whenever you have one of the world's largest banks saying they are withdrawing, it is going to have meaning."

A Citicorp spokesman said there was "no connection" between Sullivan's decision to move beyond the Sullivan Principles of fair employment and better working standards and the bank's pullout. And he declined to say whether Citicorp would also dispose of its 18 percent interest in Diners' Club South Africa Ltd.

Several of the other 193 U.S. firms with direct investments or employes in South Africa declined to say whether the Citicorp pullout would affect their decisions to stay or go.

Trevor Hoskins, public relations director for the international subsidiaries of Goodyear Tire & Rubber Co., said the company planned to continue its sales and manufacturing operations in South Africa, where it is the third-largest U.S. firm. "We have always believed the withdrawal of American companies will not end apartheid and will cause hardship to our {2,471} employes there," Hoskins said.

Unisys Inc., the mainframe computer company, said it was "still monitoring our position" in South Africa. "Certainly we listen to what the Rev. Sullivan has to say," said spokesman James Kenyon. "It is a political situation. All factors are constantly in review."

In a statement, Citicorp Chairman John S. Reed said the bank company would sell its subsidiary to the First National Bank of Southern Africa Ltd. because "current constraints on Citicorp have made it increasingly difficult to meet the needs of its South African clients in a manner they have a right to expect."

The bank had denied repeatedly and recently that it was planning to withdraw from South African operations. Last fall, a spokesman said "We believe what we are doing there . . . can make a contribution to the end of apartheid."

Experts on U.S. divestment pointed out that several factors are increasing the pressure on U.S. companies to withdraw. More and more state and city governments, for instance, are passing laws to restrict investment of public funds or awarding of contracts to companies doing business in South Africa.

At last count, there were 21 states, 72 cities and 14 counties with such statutory restrictions, Cason said.

"We see that as the kind of pressure that will bring about withdrawals," he said.

Citicorp itself had faced shareholder opposition to remaining in South Africa -- 15 percent voted in favor of a pullout -- and was seeing churches and other groups withdraw their accounts from banks around the country. Citicorp's increasing interest in expanding its banking operations nationwide also may have induced the bank company to be more responsive to public pressure, experts suggested.

"I think it's pretty clear that in Citibank's case their South African connection had become unprofitable, that it was directly affecting their bottom line," said Tim Smith, executive director of the Interfaith Center on Corporate Responsibility, a coalition of church groups opposed to apartheid.

And Sullivan's decision to turn away from the principles that for 10 years had provided a moral basis for U.S. companies to operate in South Africa was considered likely to increase public pressure on those companies to divest.

The pace of pullouts has quickened this year: There have been 23 so far, plus 16 announcements of planned withdrawals. In 1986, 50 U.S. companies ceased doing business in South Africa and in 1985, 40 companies pulled out, according to the Investor Responsibility Research Center.

Major U.S. firms that have already pulled out General Motors Corp., Exxon Corp., Honeywell Inc., International Business Machines Corp., Carnation Co., VF Corp., General Electric Co., Blue Bell Inc., Computer Sciences Corp., City Investing Corp., Eaton Corp., Bell & Howell Co., Phillips Petroleum, GTE Corp., CBS Inc., American Airlines and Martin Marietta Corp.