In a letter dated Monday, Buffett told Bill and Melinda Gates that the first donation of Berkshire Hathaway Inc. stock would go to the foundation, which has assets of $29.1 billion, next month.

"I greatly admire what the Bill & Melinda Gates Foundation ("BMG") is accomplishing and want to materially extend it future capabilities," Buffett wrote in the letter to Bill and Melinda Gates. "Accordingly, by this letter, I am irrevocably committing to make annual gifts of Berkshire Hathaway 'B' shares throughout my lifetime for the benefit of BMG."

Until now, all the money given away by the Gates Foundation has come from Bill and Melinda Gates.

The foundation spends money on world health, poverty and increasing access to technology in developing countries. In the United States, it focuses on education and technology in public libraries.

The money from Buffett comes with a significant catch. The letter says Buffett wants all his money to be distributed in the year it is donated, not added to the foundation's assets for future giving. The foundation gave away $1.36 billion in 2005, so the Buffett commitment would effectively double its spending.

He is giving the foundation a few years to get used to the idea of giving away a lot more money.

"I expect there will be a ramp-up period of two years during which this condition will not apply. But beginning in calendar 2009, BMG's annual giving must be at least equal to the value of my previous year's gift plus 5 percent of BMG's net assets," Buffett wrote.

A statement issued Sunday by the Gates Foundation and signed by Bill and Melinda Gates spoke of their relationship with Buffett over the past 15 years and his influence on their philanthropy.

"Warren has not only an amazing intellect but also a strong sense of justice. Warren's wisdom will help us do a better job and make it more fun at the same time," they said.

Bill and Melinda Gates said they were "awed" by Buffett's decision.

"And we are humbled that he has chosen to direct a large portion of it to the Bill & Melinda Gates Foundation," they said.

The Buffett pledge also requires that Bill and Melinda Gates remain alive and active in the policy-setting and administration of the foundation.

Bill Gates announced earlier this month that he would be stepping back from his day-to-day responsibilities at Microsoft Corp. in July 2008 so he can spend more time on the Seattle-based foundation. The foundation followed his announcement by saying Melinda Gates would also be taking a more active role in their philanthropic work.

Buffett said in an interview with Fortune magazine that the timing of the two announcements - one week apart - was just "happenstance."

Buffett said he would be making his announcement this week if Bill Gates continued to work full-time at Microsoft, but he said he was happy the Gateses would be devoting more time to the foundation.

The foundation is considered a leader in international public health, particularly in the fight against HIV, malaria and tuberculosis.

Buffett wrote in his letter to Bill and Melinda Gates that he hoped the expansion of the foundation would be in depth rather than breadth.

"You have committed yourselves to a few extraordinarily important but underfunded issues, a policy that I believe offers the highest probability of your achieving goals of great consequence," Buffett said.