Salomon Brothers, joining with a half-dozen major banks and securities firms, said today it plans to buy a New York-based government securities brokerage firm in a move company officials say will slash commission fees in the highly concentrated industry by as much as 50 percent.

Salomon's managing director, Thomas Strauss, said the acquisition of PGB Securities, a relatively small broker, would inject new competition into a market that has been booming in recent years because of high federal deficits.

The six other members of the consortium buying PGB are Citicorp, BankAmerica Corp., First Boston, Goldman Sachs, Merrill Lynch and J. P. Morgan & Co. Inc. The purchase price for PGB, which is owned by the New York-based investment firm of Mabon, Nugent & Co., was not disclosed, but Strauss described it as "modest."

The market in U.S. government securities has been exploding as the U.S. Treasury has sought to finance the spiralling federal debt. Volume has increased six-fold in the past six years, with daily trading reaching between $70 billion and $80 billion this year.

But commission fees charged by brokers, who trade government securities among dealers, have stayed constant, according to Strauss. In part, this is because of the relatively small number of players in the business. Although there are some 36 so-called "primary" dealers who underwrite and trade federal securities, only about six firms act as brokers among the dealers. These brokers allow the dealers to swap securities anonymously, without disclosing their deals to competitors.

Strauss said that total commission fees collected by these brokerage firms come to about $250 million a year. But he said the Salomon-led consortium planned to introduce large volume discounts that would bring down rates for the whole industry.

"A 50 percent reduction in commissions doesn't seem out of line," he said.

One New York securities broker, who asked not to be identified, agreed that the entry of such large market participants would force overall commission rates down.

"If they're willing to cut the prices by that much, then it will be like the airlines," the broker said. "Everyone will have to do it whether they want to or not."

Strauss contended that the reduced rates would promote a more efficient and streamlined marketplace that would end up benefiting the Treasury and public investors. But some questions were raised today about a potential conflict of interest among PGB's new owners, since most of them, including Salomon and Citicorp, are also primary dealers. It would be possible for Salomon, Citicorp and the others to steer all their business to PGB.

Strauss said PGB will operate independently of its owners and intends to compete for business solely on the basis of the lower commissions.