Prince George's County, under fire by black leaders for failing to meet a goal of giving 30 percent of its business to minority firms, announced yesterday it had awarded its largest-ever nonconstruction contract to Maxima Computer Systems Corp., a black-owned Rockville company.

The contract to provide computer services is valued at $15.2 million over five years plus $2.4 million in potential incentive compensation. It is also the largest agreement signed by the county with a minority firm.

The award, made public at a news conference, increases the rate of contracts going to minority firms this year from 17 percent to about 20 percent and increases the total value of goods and services purchased from minority businesses to $14 million, up from $7 million two years ago, said Dennis Brownlee, administrator of the county's Minority Business Enterprise Program.

The award was made under competitive bidding rather than through a set-aside program where only minority-owned companies submit proposals. Of the six companies that bid on the computer contract, all but one were minority-owned businesses.

Critics of the county's minority business program applauded the award to Maxima but cautioned that the Rockville-based company, with 1,400 employes and sales of $41.5 million last year, is not indicative of most of the 5,762 black-owned businesses in the county.

"This is isolated because there is not a significant number of firms that can bid on $20 million contracts," said June White Dillard, president of the National Business League of Southern Maryland.

Black political leaders and black business groups such as the National Business League are urging County Executive Parris Glendening to develop sheltered markets in the procurement system to help minority firms compete for county contracts.