An investment company controlled by the wealthy Bechtel family of San Francisco announced today that it will buy a majority interest in the old-line investment banking firm Dillon, Read & Co. Inc.

Sequoia Ventures Inc. said it will purchase all of the Dillon family holdings of the 150-year-old Wall Street house and put in "a substantial additional equity investment."

C. Douglas Dillon, the former Treasury secretary whose family name has been associated with the firm for 60 years, will remain with Dillon as a director. Three officials of Sequoia -- including another former Treasury secretary, George P. Shultz -- will join the board of Dillon.

Shultz is chairman of The Bechtel Group Inc., one of the largest construction and engineering firms in the country. He is vice chairman of Sequoia, whose chairman is Stephen D. Bechtel.

Sequoia is the private investment arm of the rich construction family, although a few other individuals such as Shultz have a piece of the firm. Sequoia so far has invested in real estate and oil and gas ventures.

Sequoia, Bechtel and Dillon are all privately owned, and the three companies refused to release either the terms or value of the transaction, which they said should be finished in early June.

Stockholm's largest bank, The Skandinaviska Enskilden Banken, also owns a portion of Dillon Read, and the banks's two representatives will remain on the board.

Nicholas Brady, a managing director of Dillon, said that although Sequoia will have the controlling interest in the Wall Street firm, management will stay in place and nothing else will change. "We'll continue exactly as we are," he said in an interview.

The purchase is the third major buyout of a Wall Street firm in the last five weeks. In March the nation's biggest insurance company, Prudential, bought the Bache Group Inc., for $385 million, and last week American Express Co. made a friendly, $900 million bid for Shearson Loeb Rhoades Inc. which is expected to succeed easily. Shearson is the second biggest brokerage house in the country, and Bache is the eighth-biggest.

Shultz noted in a telephone interview from San Francisco that Sequoia's investment in Dillon is different than the other two major acquisitions, which have been heralded as the shape of things to come in the nation's financial industry.

The Bache and Shearson takeovers were done by large financial services companies seeking to broaden the kinds of products they can offer and the network of clients they serve. Similar mergers are expected in the future as the distinctions among financial companies -- brokers, insurance companies and, if federal regulations are changed, banks -- blur.

Bache and Shearson already offer a broad spectrum of client services -- from traditional stocks and bonds to commodities and options and investment banking.

Shultz said the two companies are aware of the potential conflict of interest that could occur if either had confidential information about another's clients. "We discussed that and decided that there would be no interaction on that level at all," Shultz said.

In another development, a National Steel Corp. spokesman said that Gulf & Western Industries Inc. informed National Monday that G&W bought slightly more than 5 percent of National's stock, but that the acquisition was "for investment purposes only."