Metro officials proposed rules yesterday aimed at curbing awards of contracts to companies that do business in South Africa.

"We want to discourage apartheid," said Prince George's County Council member Richard J. Castaldi, chairman of a Metro subcommittee that is considering the issue. "We want to send people we deal with a loud and clear message."

Under the proposal, the transit authority would be required to investigate companies with ties to South Africa before awarding contracts for subway construction, equipment purchases and other services and supplies.

Any company linked to South Africa would have to show that it complies with "affirmative action" and "nondiscrimination" policies embodied in a code of conduct known as the Sullivan Principles.

The voluntary code, drawn up by the Rev. Leon H. Sullivan, a Philadelphia civil rights activist, is designed to promote desegregation, equal pay and job advancement for blacks in South Africa. More than 100 major U.S. companies with operations in South Africa are signatories to the principles.

If an investigation by Metro officials found that a company or any of its subsidiaries or affiliates failed to comply with the Sullivan Principles, the transit agency could refuse to award a contract to the firm, even if its bid were the lowest.

The proposal stemmed from a controversy last year over a contract to excavate subway tunnels for Metro's long-delayed Green Line beneath the Anacostia River. The Metro board awarded the contract after concluding that a partner in the venture had "very remote" ties to South African businesses.

At the same time, Metro officials called for adopting rules to deal with similar disputes in the future. The new plan parallels efforts to restrict financial dealings with South Africa in Congress and by state and local governments throughout the country, including the District.

The Metro board's subcommittee on minority affairs delayed action on the proposal until later this month after D.C. City Council member Hilda H.M. Mason (Statehood-At Large), a sharp critic of South Africa, called for further review in an attempt to tighten the restrictions.

"I don't want anyone to slip out," she said. "We need some definitions."

Although the transit authority awards contracts worth hundreds of millions of dollars annually, the proposed rules are expected to have a relatively small impact on most companies, officials said.

Aside from the Anacostia tunneling project, no other construction company recently hired by Metro is known to have links to South Africa, officials said. In the last year, Metro awarded $4.5 million in contracts for supplies and services to 30 companies with South African ties, officials said, but all these firms have signed the Sullivan Principles.

In another development yesterday, Metro officials recommended revised plans for excavating tunnels for the Green Line east of the Fort Totten station in Northeast Washington in an attempt to reduce disruption in nearby communities. The modified plans, which have drawn objections from some community leaders, are expected to be considered by the Metro board May 23.