Another giant Wall Street merger was announced late yesterday as Merrill Lynch & Co. said it was acquiring White Weld Holdings Inc. for $50 million in cash.

For Merrill Lynch, parent of the country's largest brokerage firm, this is the first time it has expanded through the acquisition of another brokerage.

A spokesman for the Justice Department indicated that either the department's antitrust division or the Federal Trade Commission would investigate the merger for any anticompetitive implications. Investigations of mergers of this size are routine.

The merger is expected to significantly expand Merrill Lynch's already strong position in investment banking an underwriting and provide it with access to a large European client list by virtue of White Weld's tie-in with one of the "Big Four" Swiss banks, Credit Suisse, and White Weld's strong position in the Eurodollar bond market.

White Weld & Co., with 23 domestic and 7 overseas sales of fices, will be merged into Merrill Lynch, Pierce, Fenner & Smith with 270 domestic offices and a total of 370 around the world.

The Merrill Lynch Capital Markets Group will meanwhile be renamed the Merrrill Lynch-White Weld Capital Market Group, reflecting the investment banking expertise of the acquired firm.

Of White Weld's 2,000 employees, most of them are expected to continue with Merrill Lynch after the merger. There was no immediate announcement, however, of what titles White Weld chairman Paul Hallingby Jr. and other officers will assume in the new combination.

As part of the transaction, Merryl Lynch & Co. acquired a minority interest in Societe Anonyme FInanciere de Credit Suisse et de White Weld, a company spokesman said.

Merill Lynch said it gave an option to Credit Suisse to buy this interest. It will be studied by Credit Suisse and the management of White Weld. "A decision will be made shortly as to the disposition of this interest," Merrill Lynch said.

Merrill Lynch is by far the country's largest brokerage and investment banking firm, more than double the size of its nearest publicly held competitor, E. F. Hutton & Co. In 1977 Merrill Lynch reported earnings of $43.9 million on revenues of $1.2 billion.

Because White Weld was privately held, it did not publicly report its earnings. An official at the firm said the holding company was in the black in 1977, but declined to comment on the results of the brokerage subsidiary.

White Weld tried to diversify its business two years ago when it acquired G. H. Walker, Laird & Co., a retail firm with distribution ability, but the merger reportedly did not do much to help the old-line investment banking firm.

The Merrill Lynch acquisition of White Weld is the latest in a series of major consolidations on Wall Street that has seen Lehman Brothers acquire Kuhn Loeb and Dean Witter merge with Reynolds Securities.

However, because of its size, Merrill Lynch had always been considered out of the running in the Wall Street merger derby.