A rule revision by the Internal Revenue Service extending tax exemptions to foreign investors in mortgage-backed securities could attract new money to the U.S. mortgage market, real estate industry leaders say.

The Tax Reform Act of 1984 repealed the 30 percent withholding tax on interest paid to foreigners for many types of investments, but did not specifically include mortgage-backed securities. The temporary and proposed rule issued recently makes clear that foreign investors buying securities backed by pools of home mortgages do not have to pay the withholding tax, according to Ann Fisher, an IRS attorney.

Some financial industry sources said millions of dollars worth of securities are being prepared for sale abroad as a result of the rule change, but others said drawbacks remain that could discourage sales.

Repeal of the withholding tax represents "a significant potential benefit" for the Federal Home Loan Mortgage Corp., said Leland Brendsel, acting president and chief executive officer of the corporation.

Freddie Mac has between $100 million and $200 million worth of multifamily housing mortgages that it would like to use to back securities targeted at European investors, Brendsel said. "If everything comes out right and we feel . . . it would be in the best interest of Freddie Mac, we could issue the securities in Europe in the next few weeks," he added.

European brokerage firms already selling securities issued by the Government National Mortgage Association have said the new tax exemption will add "substantially" to the attraction of that organization's securities, according to Glenn Wilson, president of Ginnie Mae. One broker "thought it might double our sales overseas," he added.

Ginnie Mae, a federal program that is part of the Department of Housing and Urban Development, guarantees securities based on pools of mortgages underwritten by FHA, the Veterans Administration and the Farmers Home Administration.

One aspect of the tax exemption remains unclear, according to Rebecca Boyd, vice president for mortgage-backed securities at the Federal National Mortgage Association (Fannie Mae). Under the new rule, securities issued after July 18, l984, are covered by repeal of the tax, but any mortgage in the pool that was originated earlier could disqualify the entire pool, she said.

"I don't know how the IRS would deal with this question," Fisher said, adding that the IRS would not make a ruling unless asked by an issuer of securities. "It may not make any practical difference," she said. "I believe, based on calls I'm getting, that people won't try" to issue securities on pools containing mortgages that pre-date July 18, l984.

Many agree with George Anderson, Ginnie Mae's director of corporate planning, who said "it wouldn't be wise to take a chance" on putting older mortgages in pools.

Fannie Mae and Freddie Mac are major sources of funds for the housing market because they buy mortgages from lenders, who in turn can use the money to originate more home loans, and sell securities based on the mortgages in the secondary market.

"While we're delighted to see this kind of barrier the withholding tax removed for mortgage-backed securities, there are other barriers to overcome," Boyd said. "I don't think there'll be an avalanche" of foreign investment in the securities, he continued.

The requirement that securities must be registered in the name of the holder discourages many investors, who prefer not to be identified because they hope to circumvent tax and currency restrictions in their home countries.

A second drawback is the system of making monthly payments to investors in U.S. currency, subjecting them repeatedly to swings in exchange rates.

"We've got to come up with ways to overcome the monthly pass-through," Boyd said. "An instrument with a semiannual payment would be a first step. And we must develop financial tools to take care of the currency exchange risk."