VOSLOORUS, SOUTH AFRICA -- A year ago, Ronnie Mlonyeni had a hardscrabble existence, repairing old cars in the tiny back yard of his home in this crowded black township and barely making enough money to feed his wife and two children.

Work was sporadic, and if he could pull in the equivalent of $200, Mlonyeni would consider it a good month.

Now Mlonyeni is the owner of a thriving used car and repair business in a new industrial park on the edge of his township, making a monthly profit of $1,000 to $1,500 and still growing. He employs six mechanics and assistants and recently won a contract to repair municipal vehicles.

"When I was working at home, nobody knew me. Now I can't keep up with the work. I need more room and more mechanics," said Mlonyeni, who was largely self-taught in car repairs.

Justice Radebe, 45, may make it even bigger.

He ran a small grocery shop in Vosloorus that, while moderately profitable, faced stiff competition from white-owned supermarkets in nearby Boksburg, 23 miles south of Johannesburg.

Now he runs his own sprawling supermarket with 44 employes, and predicts sales of $250,000 a month. Although he opened just last month, Radebe's computerized inventory tells him he could make a 17 percent pretax profit, which by township standards would make him a very wealthy man.

Mlonyeni and Radebe represent a small but rapidly growing segment of South Africa's 26 million black majority -- entrepreneurs who are fighting their way into the mainstream of the country's economy in the midst of political and social turmoil and in spite of apartheid laws that place them at a strong disadvantage to their white competitors.

They are doing it with a major boost by the Small Business Development Corp., a public company owned jointly by private industry and the government but whose board of directors is controlled by the private sector.

Blacks hold 50 percent of the purchasing power in South Africa but own only 2 percent of the assets, and it has become an axiom of most white businessmen that the only hope for the country politically and economically is to raise the economic level of blacks and create greater overall wealth.

It is on this principle that the SBDC, roughly equivalent to the Small Business Administration in the United States, has been bankrolling a growing number of black entrepreneurs, providing seed money or low-cost working space to businesses ranging from shoeshine operations to steel fabrication factories.

Although linked to the government that created the apartheid policy of racial separation and that suppresses by force black aspirations for majority rule, the SBDC surprisingly has not been a target of boycotts or attacks by militant blacks.

In fact, the corporation often skates close to the edge of legality, looking the other way sometimes when its clients fail to obtain required licenses or when they violate the Group Areas Act, which strictly segregates residential and business areas by race. That willingness to circumvent apartheid laws appears not to have been lost on militant blacks who would normally oppose any institution allied with the government.

"We don't break the law, we just push it to its limits," James Scott, SBDC general manager in the central region, said in an interview.

Recently the corporation established 17 Indian traders in Bloemfontein, the first Indian businessmen ever to operate in that capital of the conservative Orange Free State. Controversy arose when the merchants and their families began living in the back of their shops in violation of residential segregation laws.

In its six years of existence, the SBDC has approved more than 16,000 direct startup loans to entrepreneurs, the majority of them black, for a total of $190 million or an average of $12,500.

In terms of amounts loaned, white entrepreneurs, who generally seek greater amounts in order to start bigger businesses, got the larger share. But Scott said the trend now is for blacks to seek bigger loans than before.

In addition, the SBDC's property development project has completed construction of about 2,700 stores and factory units at a cost of $74 million. The space is rented to the fledging small businessmen for about half what they would pay in an urban market.

The corporation estimates that its assistance has created more than 87,000 jobs and maintained another 70,000 that would have been lost to failure without assistance.

Scott noted that until 1979 -- when black ownership of land on 99-year leaseholds was introduced, along with the acceptance of the permanence of blacks on land outside the tribal "homelands" -- black business had little hope for growth.

"It has taken some time for those inhibitions to wear down, but a whole new era has dawned on black entrepreneurship. Grassroots entrepreneurship is creeping out all over, and we are merely being a catalyst for it," said Scott.

Aspiring black entrepreneurs can walk into an SBDC office and after filling in a simple, three-quarter page application, can walk out the same day with a "miniloan" of $1,000 to $2,500. Or, Scott said, they can go through a more complicated process seeking $200,000 or more for a high-technology venture.

They can also seek factory space, at favorable rents, in one of a number of industrial parks that the corporation has built near black townships throughout South Africa, using capital invested by scores of white-owned companies that are shareholders in the corporation and supplemented by government grants.

Other entrepreneurs merely walk in seeking advice on how to form a corporation, obtain licenses, register workers and pay taxes.

In Mlonyeni's case, he won approval to rent 4,800 square feet of factory space in the SBDC's Vosloorus industrial park for $400 a month and started his business with virtually no capital.

Radebe rented his sprawling supermarket space in a new 65-shop SBDC-built shopping center for a bargain $6,000 a month and, with a $300,000 commercial bank loan and $30,000 of his life savings, installed fixtures and counters. He stocked the store with a 30-day payment leeway from suppliers and has been doing a thriving business since.

The SBDC shopping center in Vosloorus is the biggest in a black urban area in South Africa, built at a cost of $2.8 million. Already, 51 shops, ranging from fast food to clothing stores, are being run by blacks.

Scott said the biggest problem facing the new black entrepreneurs is convincing blacks from the township to change their longstanding habit of shopping at white-owned stores in Boksburg or Johannesburg.

The political unrest and the state of emergency imposed in June 1986 have not significantly affected black entrepreneurship, Scott said.

"Most black business people are guys who want to get on with the job, and even if fires are burning around them they want to do business," he said.

He added, "Years ago, there were a few privileged people who had trading licenses and acted as tycoons in their areas. Now, more and more black entrepreneurs are becoming involved, not just an isolated few who could be easy targets for radicals."

And in the true spirit of capitalism, many of the fledging black entrepreneurs become protective of their investment.

When asked whether he would allow his shop to become unionized, Mlonyeni, a self-described political militant, rolled his eyes in chagrin and replied, "No, no, I don't think so."