Cranking up an already-hot competition one more notch, T. Rowe Price Associates, the $9 billion mutual fund folks from Baltimore, recently jumped into the discount brokerage business.

Promising to save customers "up to 75 percent on the commissions charged by full-cost brokers," the firm used its customer mailing lists -- 500,000 households that have invested in its 12 money funds -- to solicit business for its new discount operation.

To run the new operation, T. Rowe Price hired Maurice Minerbi of Potomac, who spent five years managing the Washington office of Charles Schwab & Co., one of the nation's biggest discounters.

Minerbi, who sounds pleased with the way things are going, said that the company opened more than 5,000 brokerage accounts in the first 60 days and has four brokers on the job, handling about 35 trades a day. The firm's goal for its first year was to open 10,000 accounts. Thus, Minerbi said, the company is way ahead of projections.

Looking before they leaped, T. Rowe Price officials first did some research. They found that two-thirds of their shareholders expressed interest in using their discount service. Half of that group said they already dealt with discount brokers. Deciding the prospects looked good, company officials worked up advertising literature and stuffed it in the quarterly statements sent to their mutual fund shareholders.

While the emphasis in the advertising was on the "up to 75 percent" discount, T. Rowe Price's chart shows that an investor would save that much only on a large volume trade. The example is for a trade of 2,000 shares at $5 a share. The customer would pay an $80 commission instead of what the firm says would be the $322 charged by a "typical full-cost broker."

On the other end of the scale, on a 100-share trade at $25, the investor would pay T. Rowe Price a minimum $40 commission compared with the "full-cost" fee of $59 -- a savings of only 32 percent.

The literature makes no comparison with other discounters, but Minerbi said that he considers his fees to be competitive.

As with other discounters, T. Rowe Price brokers will only buy and sell for a customer. They offer none of the advice or research that full-service brokers provide.

For the moment, managers at Quick & Reilly and Charles Schwab & Co., two of the biggest discount firms in the Washington area, seem not to be worried about the new competition.

John Lanza, manager at Quick & Reilly, said it was too soon to feel any competition from T. Rowe Price. In any event, he said, "our competition is with the big firms."

T. Rowe Price's new role as a discounter might even help give his business "a better name" -- adding to the aura of respectability that once eluded discounters, Lanza said. When Lanza began working in the discount firm eight years ago, he recalled, "People were very suspicious of us. They're not anymore." Lanza's office has 17,000 accounts.

At Schwab, manager Scott Frazer said he didn't expect much competition, either. For one thing, he noted, T. Rowe Price is directing its solicitations to its current shareholders, rather than to the general public. Until that changes, Frazier said, the impact of T. Rowe Price on other discounters will be limited. The local Schwab office has 21,000 accounts.

At the former offices of Laidlaw Adams & Peck Inc., in Rockville, Tysons Corner and Dupont Circle, phones are now answered with the greeting: "Laidlaw Ansbacher."

The new name reflects the recent merger of Laidlaw, a New York-based investment firm, with Henry Ansbacher Holdings PLC, an international merchant banking organization with headquarters in London. The firms have combined assets of $350 million, net worth of $70 million, a staff of 780 and offices around the world.

What will the merger mean to brokers in Laidlaw Ansbacher's local offices? John R. Moysey, manager of the Rockville branch, largest of the three offices, predicted an expansion of the firm's sales force, office space and research activities.

Virginia's United Savings Bank has raised $3.6 million in a private sale of 197,781 common shares at $18 each to a group of 14 investors and bank customers. Last May, the bank raised $4 million in a private sale of 330,000 preferred shares at $12.25 each to about 100 depositors. Bank President Howard D. Orebaugh said the sale of stock will help the bank stay well-capitalized during a period of rapid growth.

As with many thrifts, the last two years have seen a major turnaround at United Savings, which lost about $750,000 in 1982 but earned about $750,000 in 1983. Earnings for the first nine months of this year were $2.56 a share, compared with $1.59 for the same period last year.

But the good news was tempered by the figures for the three months ending Sept. 30, in which earnings fell from $1.07 to 99 cents per share. Chief Financial Officer Chris Burch attributes the drop to the effect of rising interest rates on the operations.

Meanwhile, bank assets have grown from $75 million to $217 million in 2 1/2 years and, in the months to come, United Savings will expand from six offices to eight, moving its headquarters from Vienna to Tysons Corner.

The Math Box Inc. of Rockville has sold 750,000 shares at $8.75 each. The company sold 600,000 of those shares, while stockholders sold the remaining 150,000. Outstanding shares now total 2.9 million.

The firm, which sells microcomputer systems in the Washington and Philadelphia areas and trades over the counter, also said it will open four new computer centers in the next two months. Johnston, Lemon & Co. and Wheat First Securities Inc. helped manage the underwriting for the OTC stock.

Software AG Systems Inc., a Reston firm that develops advanced information systems software products, took a look at its books the other day and decided to buy back $5 million worth of its own stock.

"Very frankly and honestly," said Gil Markbein, treasurer and controller of the company, "We had a surplus of cash. . . . We felt the company's stock was undervalued and we felt it was a good investment."

The company won't do anything with the stock, he said. In fact, it will be deregistered. He estimated that it would take two to three months to buy back the estimated 350,000 shares, with Salomon Brothers handling the purchases of the OTC stock.

Software AG stock closed Friday at $14.75 on volume of 85,300 shares, up $2.125 for the week. Over the past five years, the stock's price has ranged between $5.125 and $22.75.

Moody's Investor Service has raised its rating for Washington Gas Light Co.'s seven publicly held first mortgage bonds from A2 to A1, and raised its ratings on the company's six publicly held issues of preferred stock from A2 to A3. Fitch Investors' Service Inc. raised its ratings on the first mortgage bonds from A to AA-, and went from A- to A on the preferred stock. The ratings changes were based on the company's improved financial condition, a spokesman for the utility said.