You ask why, particularly, the United Negro College Fund should be honoring Leo M. Bernstein, a Washington banker, at its 40th anniversary celebration tonight?

For the answer, or part of it, you have to go back to 1933. Washington, not yet galvanized by the New Deal, is still a sleepy southern town where the old social order rules unchallenged. Real estate contracts contain covenants, dignified-sounding documents that contain ugly words: If you are "colored" or "of Hebrew origin" you cannot buy property in this area or that area. And the areas are actually marked on secret maps.

Here comes a Howard University professor named Proctor, a black man married to a white woman, and he wants to buy a house at 613 Kenyon St. NW, just three blocks from the university, in a whites-only neighborhood. A real estate agent, 19 years old, sells him the house. The sale is disputed in what becomes known as the Proctor Case, and it is won by Proctor and his lawyer, a man named Henry Lincoln Johnson.

Johnson asked his opponents one simple question: "Can you prove you are white?"

Tonight Johnson, now aged 80, will introduce that teen-age realtor, now 69: Leo M. Bernstein. "I didn't even have a license then," Bernstein says, interviewed in his office at Security National Bank, where he is chairman of the board. "In those days morticians and realtors didn't need licenses. But I had read those covenants, and I said, 'That's not fair.' "

He had started at 18 at the Washington Rental Co., finding tenants for the city's many vacant homes and apartments and getting paid half the first month's rent. He bought 2,000 embossed business cards for $1.50, office furniture for $30. His first phone bill came to $3.50. He started selling houses to people for down payments of $500 or less, guaranteeing mortgages long before the government thought of it.

He did so much business that he had to record sales on a blackboard as the crowds milled in his office, buying and selling. "I handled 10,000 sales in 51 years," he recalls, "and I never lost 5 cents."

Going into run-down Georgetown, he started buying rattletrap houses for a few hundred dollars, fixing them up and selling them by the hundreds. There were some heartbreakers: One house he had sold for $500 later went for $75,000 and is probably worth six figures by now. But perhaps more than any other single person, Leo Bernstein pioneered the creation of today's posh Georgetown.

"People are still coming back to the city, even young families," he observes. "The next area to be developed will be around 14th and U, no question about it. After the election, prices will drop about 15 percent because they're too high right now, and the inner city will be full of bargains."

Later he merged his title firm into District Realty Insurance, and in 1948 founded Guardian Federal Savings and Loan. Taking over D.C. National Bank in the 1960s during an uproar over loans made to speculators ("We were loaning money to blacks, who were thought not to be credit-worthy"), he built up that institution, moving to Diplomat National in 1980 and later to Security. This fall he will sell his stock in Security to concentrate on the Women's National Bank.

"This city and country have done a lot for me, and I want to help other people, not just those with money," says the man whose scholarly grandfather immigrated from Lithuania to become a kosher butcher and whose father ran the city's first Army and Navy stores.

His interest in Americana takes a tangible form. In 1945 he renovated the federal city's oldest house, at 2618 K St. NW, and 15 years later he bought the crumbling 1797 Wayside Inn in Middletown, Va., and revitalized the whole town. For years he has tried to interest the District in his collection of early presidential letters and artifacts, including a lock of George Washington's hair, but it remains scattered, some of it in the lobby of the Georgetown Hotel, which he used to own.

At the moment he is negotiating with York, Pa., the first capital of the young nation, offering to donate money for a museum in the recently restored building where the first Continental Congress met.

Today Leo Bernstein lives in Georgetown with his wife Beverly. Their two sons are in real estate, their daughter a newspaper columnist. He maintains membership in five Jewish congregations, and the list of his community activities -- from the Big Brothers to the Catholic University Alumni Fund -- fills a solid page, single spaced.