The University of Maryland Board of Regents has written to 28 corporations in which it owns stock asking them what their politics are toward apartheid in the Republic of South Africa.

The firms are all multinational corporations doing business in South Africa.

Last fall, black students, faculty members and staff presented University President Wilson H. Elkins with a letter demanding that the regents halt all investments in corporations doing business in South Africa.

The Board refused to take that step, but it did say in the letter that "our Board of Regents believes that the doctrine of apartheid does not represent the basic philosophy of the citizens of the United States of America, nor does it represent the principles of the University of Maryland."

The letters to the corporations were signed by Hugh McMullen, chairman of the Board's committee on endowments and gifts.

The letter, dated April 4, asked the companies to advise the board on what they are doing to "work toward the termination of apartheid in the Republic of South Africa.

David Green, a spokesman for the university, said seven of the companies have replied. Most said they deplored apartheid, he said, and many said they had set up training programs specifically to aid blacks in South Africa.

Those companies are Caterpillar Tractor Co., E.I. Du Pont de Nemours Co., Exxon, Hewlett Packard Co., International Business Machines and International Telephone and Telegraph.

Tractor and Gamble also responded to the regents letter but it said its holding in South Africa were not extensive.

Among the other companies the University sent letters to are American Home Products Corp., Black and Decker Manufacturing Co., Burroughs Corp., Citicorp., Clark Equipment Co., Coca Cola, Eastman Kodak, General Motors and Gilette Corp.

Also included were Johnson and Johnson; Merck and Co., Inc.; Minnesota Mining and Manufacturing Co.; Motorola, Inc.; National Cash Register Corp.; Pepsico, Inc.; Charles Pfizer and Co., Inc.; Phillips Petroleum Co.; Schering-Plough Corp.; Warner-Lambert Co.; Weyerhaeser Co. and Xerox.