The world's second-richest man, Warren Buffett, became one of the world's biggest philanthropists Sunday with the announcement that he would bequeath the bulk of his roughly US$44 billion fortune to the foundation established by billionaire Bill Gates and his wife.

The decision to start giving next month through annual stock donations represents a stark reversal for the investment wizard, who for years had said his wealth would be pledged to philanthropies after his death.

Buffett's gift will radically boost the resources of the Bill and Melinda Gates Foundation, which is already the world's largest philanthropy with assets of more than $29 billion.

Earlier this month, the world's richest man and Microsoft Corp. co-founder decided to give up his daily duties at the software company in 2008 to spend more time at his foundation, which is considered a leader in international public health, particularly in the fight against HIV, malaria and tuberculosis.

Gates also serves as a board member of Berkshire Hathaway Inc., Buffett's investment conglomerate, and the men socialize regularly.

The 75-year-old Berkshire chairman and CEO had been expected to leave his vast holdings of Berkshire stock largely to the Susan Thompson Buffett Foundation, begun by Buffett and his late wife. That foundation has given millions of dollars to hospitals, universities and teachers, as well as to Planned Parenthood and other abortion rights groups.

Buffett said he plans to give away 12,050,000 Class B shares of Berkshire Hathaway stock to the foundations, but he will have to convert some of his 474,998 Class A shares to complete the gifts. One Class A share, which sold for $92,100 on Friday, can be converted into 30 Class B shares, which sold for $3,071 Friday.

The gifts would be worth nearly $37 billion based on Friday's closing share price.

Buffett's assistant Debbie Bosanek said Buffett would not be available to comment Sunday. But letters outlining the gifts were posted on the company's Web site, and Buffett explained his decision in an interview with a Fortune magazine editor, Carol Loomis. She has edited Buffett's annual letter to shareholders for several years.

Buffett told Fortune that he decided to start giving his money away now because he has been impressed with Bill and Melinda Gates and the work they've done through their foundation. And he decided it would be easier to give to a large foundation instead of trying to expand his own foundation.

"What can be more logical, in whatever you want done, than finding someone better equipped than you are to do it?" Buffett told the magazine. "Who wouldn't select Tiger Woods to take his place in a high-stakes golf game? That's how I feel about this decision about my money."

Andy Kilpatrick, a stockbroker who wrote "Of Permanent Value, the Story of Warren Buffett," called the announcement remarkable, but said he always expected something more to develop from the relationship between Buffett and Gates.

"It's Buffett and Gates merging in a way for charitable purposes," Kilpatrick said.

Buffett has long said limiting the spread of nuclear weapons is the greatest challenge facing mankind. And Kilpatrick said Buffett probably agrees with the Gateses' concerns about population control, disease and education.

In a statement, Bill and Melinda Gates applauded Buffett's decision.

"We are awed by our friend Warren Buffett's decision to use his fortune to address the world's most challenging inequities, and we are humbled that he has chosen to direct a large portion of it to the Bill & Melinda Gates Foundation," the couple said.

Buffett suggested that his children should focus their charitable resources on needs that would not be met otherwise.

"Focus the new funds and your energy on a relatively few activities in which HGB (Howard G. Buffett Foundation) can make an important difference," Buffett wrote. He included the same paragraph of advice in the letters to each of his children.

Buffett said he plans to earmark 10 million B shares for the Gates Foundation, 1 million B shares for the Susan Thompson Buffett Foundation named in honor of his wife and 350,000 shares for the three foundations run by each of his children.

Buffett plans to give each foundation 5 percent of his total pledge each year in July.

In the interview with Fortune, Buffett acknowledged that the foundations may sell off the Berkshire stock to raise cash. But Buffett said he doesn't think that will affect the price of the stock because the gifts will be spread over time.

"I would not be making the gifts if they would in any way harm Berkshire's shareholders," Buffett told Fortune. "And they won't."

But Kilpatrick said this announcement is still likely to prompt some people to sell Berkshire stock on Monday morning. Kilpatrick said he doesn't think this will hurt the company in the long run especially because it might be added to the S&P 500 index in the future, which would help the stock.

Buffett's health has been the subject of speculation. He has said a succession plan is in place at Berkshire but refuses to name a successor.

In the letters, Buffett wrote, "My doctor tells me that I am in excellent health, and I certainly feel that I am."

Berkshire owns a diverse mix of more than 60 companies, including insurance, furniture, carpet, jewelry, restaurants and utility firms. And it has major investments in such companies as H&R Block Inc., Anheuser-Busch Cos. and Coca-Cola Co.