Andrew Murstein has found a lot of green in yellow.

Translation: He’s made a fortune by expanding the taxi business that his grandfather Leon, a Polish immigrant, started in 1937 by buying one medallion for $10. Now, medallions – those tin plates that must be affixed on each one of New York City’s 13,000 taxis – are going for $400,000 apiece. Mr. Murstein owns at least 500, which he leases out, making him the biggest owner of medallions in the city.

When financial institutions decided to cut back on lending in the wake of September 11, 2001, thereby discouraging many people who wanted to rent medallions, Mr. Murstein stepped in by starting his own bank.

The bank, known as Medallion Financial, is traded on the Nasdaq (Symbol: TAXI). It is especially receptive to applications from women and minorities. Mr. Murstein’s bank also lends to taxi drivers in 12 cities, including New York, Chicago, and Boston. One out of every three taxi drivers in New York has either bought his medallion from Mr. Murstein, rented it from him, or borrowed from his bank to acquire the medallion.

His ambition was spawned very early in life. His father Alvin encouraged him to come to his office after school and on weekends to collect cash from drivers who rented the Murstein taxis.

“I quickly learned who was fibbing about lost fares and who wasn’t,” Mr. Murstein said. “At first I was susceptible to heart-rending stories of why drivers had failed to come up with cash that they owed to us. But my father told me, ‘You’ve got to stop believing stories.’ I learned that a deal is a deal. If you owe money to someone, then you’ve got to pay it.”

The lessons from those experiences were with Mr. Murstein as he studied economics at Tufts University and then earned an MBA at New York University. He thought of becoming an investment banker, and did work briefly for Bear Stearns. Then he decided to join the family business.

Was the MBA an asset in the taxi industry?

“The only thing I learned at NYU was how to play poker well,” Mr. Murstein said. “Poker teaches you how to read people and gauge their intentions. And that’s very important in business.”

But his MBA didn’t fetch him any special dispensation from his father.

“My father told me, ‘You need to start at the street level,'” Mr. Murstein said. “And so I did.”

That meant doing what he did as a child – collecting money from the company’s borrowers.

“You know, it’s always tough to work in a family business,” Mr. Murstein said. He did not intend that remark to be humorous.

Humor, however, is a quality he brought to his professional life. It has helped him defuse many a delicate negotiation. It has also won him the reputation of being tough but easy to get along with.

He is also a raconteur. Mr. Murstein recalled an auction where he’d had his mind set on an ancient Checker Cab (Those cabs have long gone out of production and from America’s cities.). The bidding started at $30,000 and rose to $170,000. At that point, Mr. Murstein bowed out, and someone else got the vehicle.

Later that day, he happened to be visiting a car shop where he had spied a Checker Cab. It belonged to a friend of the shop owner. By serendipity, that friend happened to drop by. “Would you sell that Checker?” Mr. Murstein asked. The man scratched his head, then said he would. “How much would you want?” Mr. Murstein said, the earlier auction still fresh on his mind.

The man scratched his head again.

“Oh, say, $2,000?” he said.

Mr. Murstein could hardly believe his ears.

“I bought that Checker on the spot,” he said. “The lesson? Never let a good opportunity go by.”

In the early 1990s, he saw an opportunity to diversify from the taxi business. He started expanding on his father’s lending to taxi drivers and began lending to a variety of small businesses. By 1995, his company had made $100 million in loans. By 2005, the company had made $10 billion in loans.

Mr. Murstein saw an opportunity in 1996 for creating advertising billboards on the roofs of taxis in American cities and in Japan. He invested $1 million. Last year, he sold that business to Clear Channel for $35 million.

Similarly, rather than being disheartened by some banks’ decisions to trim their lending to him after September 11, Mr. Murstein saw an opportunity to open his own bank. Since its inception two years ago, the bank, Medallion Financial, has loaned more than $850 million.

“You’ve always got to evolve,” Mr. Murstein said, but warning that expanding a business also meant being careful about proposals that come along. He evaluates some 300 business propositions each year. He came close to buying the Kansas City Wizards. Although that deal didn’t happen, he hopes one day to become a major player in sports, possibly arena football.

“Business is in my blood,” he said.