Ralph Clark retired from the D.C. fire department in 1975. Then his career began.

Clark, 57, now runs his own janitorial supply service in the Prince George's community of District Heights, not far from his Suitland home.

"I like this idea of financial independence," said Clark, one of a growing number of blacks who say that Prince George's is a good place for them to do business in the Washington area.

There are about 400 black entrepreneurs in Prince George's who are certified to do business with the county, according to county officials. That is just under 6 percent of the more than 7,000 businesses listed by the county Chamber of Commerce.

Figures for black-owned businesses generally are more difficult to obtain. But in 1977, the last year for which such figures were compiled by the U.S. Commerce Department, there were 2,347 such businesses in the county, almost a quarter of the state's total and far more than in other Washington area jurisdictions.

Nationally in 1977, there were 231,203 black-owned businesses, less than one half of 1 percent of all businesses, according to U.S. Commerce Department figures.

The percentage of black businesses within the county is growing, government, business and community leaders say, as the county's economic development base expands. The county development corporation reported recently that nearly 2.3 million square feet of new commercial space is scheduled to open in Prince George's this year, up from 1.6 million last year and 1.1 million in 1983.

Some blacks already have found a prosperous niche in Prince George's. Auto dealer Joseph McLaughlin, who died late last year, established an Oldsmobile dealership in Capitol Heights that in 1983 had sales totaling $14.5 million, according to Black Enterprise magazine.

The only area firm to surpass this mark, the magazine said, was Systems and Applied Sciences Corp., based in Vienna. That firm did $37 million worth of computer and electronic data systems business in 1983.

More modest businesses have had enough success in Prince George's to spawn a black middle class that lives and, in many cases, works in communities such as Landover, Camp Springs, Lanham, Kettering, Suitland and Oxon Hill. Many of the businesses are in the service industry -- including restaurants and hair salons. Others run the gamut from the Jack Seidel automobile dealership on Central Avenue to Consuelo Parker's consignment boutique Forever Classic in Camp Springs.

Several factors make Prince George's more attractive to minority businesses than other Washington suburbs, said Edmund Cronin, board chairman of Smithy Braedon, a Washington-based real estate firm, and past chairman of the Greater Washington Board of Trade's business development bureau. One factor, he said, is the sheer number of black residents in Prince George's.

The 1980 U.S. Census reported that 248,000 of Prince George's 665,000 residents were black, or 37 percent. And demographers have noted that black people leaving Washington since 1970 have relocated more often in Prince George's than in any other suburban jurisdiction. The average income level for these new black residents, according to demographer George Grier, is slightly higher than those found in District households.

Also, Cronin said, land prices and square footage costs are generally cheaper in Prince George's than in neighboring jurisdictions, making it easier for an entrepreneur with limited resources to start a business. A report prepared last summer on county business growth found that lease rates for commercial space in Prince George's ranged from $9 to $16 per square foot, compared to $12 to $20 per square foot for similar space in Montgomery and Fairfax counties.

"The biggest single problem minority entrepreneurs have -- all entrepreneurs have it -- is raising capital," Cronin said.

It is in the area of capital that government traditionally has intervened, in efforts ranging from the federal Small Business Administration's Minority Small Business and Capital Owner Development program to state and local incentive programs that encourage private firms to lend seed money to new businesses. The former, known as Section 8A, supplied $470 million in federal procurement subcontracts to businesses in the Washington region during fiscal 1984, according to SBA records.

But since 1980, the SBA has concentrated on weaning businesses off government support once they have established themselves as competitive. Some business people have complained that this removes crucial support from small businesses just when they need it most, but the SBA has said that limiting the term of aid is the best way for a business to learn to succeed on its own.

The Prince George's county government substantially increased its help for black-owned businesses when the County Council enacted a law last year -- the strongest in Virginia or Maryland -- that requires the county government to do 30 percent of its purchasing with minority firms.

"I want to get across the idea that the more others benefit, the more I do," Prince George's County Executive Parris Glendening said recently. "There is a misconception that the pie is of a limited size . . . . My intention is to see that anybody out there who wants to make a million in this county can ."

County records show that in the fiscal year that ended last July, 10 percent of the nearly $45 million the county spent in purchases went to minority firms, including companies headed by women, Hispanics and Asian Americans. Most of that money was paid to trash hauling companies.

"Most of our businesses are service businesses," Clark acknowledged. "There's nothing wrong with that, but why can't we get into [growth businesses]?"

W.O. Stephens, a black Oxon Hill-based engineer, made his jump into a growth industry when he opened a high-tech firm in Prince George's six years ago. But he started seeing the county as a potential market only a short while ago, he said.

Unlike some of his black colleagues, his Stephens Engineering Co. literature makes no mention of his race. Large firms in the private sector that have nothing to gain by filling affirmative action quotas often react negatively to a black-owned business, he said, although he believes that attitude is changing.

Stephanie Colbert Hopkins, 36, who has owned a public relations firm for 10 years, said that she sometimes has trouble as a black female getting large white-owned firms to consider her seriously as a potential subcontractor despite her company's successful track record.

"I happen to be a woman who happens to be black," she said. "Those things are secondary to me, but I find that with the people I meet it becomes primary."

The way to address these problems, said Clark, is through tenacity and common business sense. "I know that blacks are consumers," said Clark, who opened his janitorial business with $300 he borrowed from his mother in 1968 after selling soap products "virtually door-to-door" part-time.

Clark said he advised his four children to go into business for themselves, and he gives the same advice to students ["our little brothers and sisters," he calls them] when addressing groups at Bowie State College.

He also advises black people to crack the political system.

"There is no such thing as being a successful businessman and being apolitical," said Clark, who is chairman of the National Business League of Southern Maryland, a director of the county's Economic Development Corp. and a member of the state Chamber of Commerce, the NAACP and the county fair association. Clark was also appointed by Glendening to serve as chairman of the minority procurement task force that drew up the new county law.

The Prince George's County law mandates that, in addition to the 30 percent purchasing quota, 20 percent of the county's subcontracting work be awarded to minority-controlled companies. However, loopholes remain in the government's procurement practices. The Washington Suburban Sanitary Commission recently beefed up its minority procurement program, setting a goal of 25 percent minority involvement in all contract awards. But the county's Board of Education, which handles procurement for the entire school system, makes no effort to contract out its services to minority-owned firms, according to school board personnel.

The District purchasing program, which Graves praised as a model, requires that D.C. government agencies reserve 35 percent of their purchasing contracts specifically for minority firms. Montgomery County requires bidders on contracts of $100,000 or more to share 10 percent of the contract with minority firms. Officials in Fairfax and Arlington counties and in Alexandria said that they have set no percentage goals but sponsor outreach programs for small and businesses.

Business assistance programs can hinder independence, said Clark, noting, "As long as you've got crutches, you'll never know how to walk."

Nonetheless, the county's procurement policy draws strong endorsements from Clark plus the National Business League and the Metropolitan Trade Association, two county-based groups made up mostly of black business people. "We want a fair chance at the market," said Wilbur Wilson, director of the Metropolitan Trade Association.

A recent report on county business commissioned by the Prince George's Economic Development Corp. said that the county has "an obligation" to assist minority business.

The problems faced by minority businesses just starting up are not unique, said Joseph Edwards, president of the county's Economic Development Corp. Edwards also believes race is no longer a negative factor in starting a business. "Ten years ago," he said, "I would have told you something different."