WASHINGTON (AP) _ The Supreme Court today limited the power of an securities regulatory agency to protect customers of brokerage firms.

The court said the Securities Investor Protection Corporation (SIPC) wrongly invoked an anti-racketeering law in its 1983 efforts to recover money it paid to customers of two brokerage firms gone bust.

But the court left unanswered a key question: Whether the agency or individual investors who claim they were victimized by stock manipulation while they held their shares ever may use the law to sue brokers.

Four of the nine justices voted to allow such suits under the Racketeer Influenced and Corrupt Organizations Act (RICO).

But Justice David H. Souter, writing for a five-member majority, said there was no reason to address that issue in the case before the court.

Past Supreme Court rulings have limited the anti-fraud provisions of federal securities laws to the purchase and sale of stock. Shareholders who rely on phony information in deciding to keep their shares and lose money as a result cannot sue under those laws.

Federal appeals courts have split in deciding whether such investors may invoke RICO in suing to get their money back, and those conflicting rulings went unresolved today.

But the decision was a victory for Robert Holmes, one of 75 defendants sued in 1983 by the SIPC after the failure of two brokerage firms, First State Securities Corp. and Joseph Sebag Inc.

Writing for the entire high court, Souter said the alleged conspiracy to manipulate stock was not tied closely enough to the injuries suffered by investors to support a RICO lawsuit.

Justices Sandra Day O'Connor, Byron R. White, John Paul Stevens and Antonin Scalia voted to allow RICO suits by investors who neither bought or sold stock because of the alleged fraud.

The SIPC suit against Holmes alleged that he conspired to manipulate the stock of six companies traded in the over-the-counter market. He and his co- defendants were accused of defrauding investors of $14 million between 1964 and 1981.

The SIPC was created by Congress in 1970 after the collapse of numerous brokerage firms. Its purpose is to provide greater protection to customers and dealers, and to members of national securities exchanges.

The case is Holmes vs. Securities Investor Protection Corp., 90-727.