SEATTLE - SEATTLE - Forty wealthy families and individuals have joined Microsoft Corp. co-founder Bill Gates and billionaire investor Warren Buffett in a pledge to give at least half their wealth to charity.

Six weeks after launching a campaign to get other billionaires to donate most of their fortunes, the chairman and chief executive of Berkshire Hathaway Inc. released the first list yesterday of people who have signed what he and Gates call the "giving pledge."

Buffett said he, the Gateses and others have made 70 to 80 calls to some of the nation's wealthiest individuals.

"We're off to a terrific start," Buffett said, adding that he'll keep pressing the billionaires who rebuffed his request.

New York City Mayor Michael Bloomberg and two Long Island couples were on the pledge list - financier and Home Depot co-founder Ken Langone and wife Elaine, and well-known mathematician and hedge fund founder James Simons and wife Marilyn, who's president of the Simons Foundation, which supports academic research.

Both couples are known for their philanthropic commitments.

Two years ago, Simons donated $80 million to Stony Brook University, the largest single gift ever given in the state's university system, to create the Simons Center for Geometry and Physics on the campus. He had been chairman of the mathematics department there until 1976, when he left to become a financier and later started Renaissance Technologies Corp., a hedge fund manager that hired astronomers and other scientists to develop what turned out to be highly successful trading strategies.

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Roslyn-born Langone was just in his mid-20s when he began rising quickly in the business world and joined Texas businessman Ross Perot to start up Electronic Data Systems, which went global. Langone also founded an investment bank and led a high-profile but failed effort in 2005 to buy the New York Stock Exchange. The venture capitalist is better known as co-founder of Home Depot and has donated millions of dollars to various causes, from cancer research to Home Depot's Ken's Kids.

Neither billionaire could be reached to comment Wednesday afternoon. Simons, 72, of Old Field, was in the South Pacific. Langone, 74, of Sands Point, was tied up in an earnings report conference.

Gates and Buffett estimate their efforts could generate $600 billion in charitable giving. In 2009, American philanthropies received a total of about $300 billion in donations, according to The Chronicle of Philanthropy.

Bloomberg, who has a fortune estimated by Forbes magazine at $18 billion, said his whole family is in tune with his giving plan.

"I've always thought your kids get more benefit out of your philanthropy than you will," he added.

With Ellen Yan