WASHINGTON – Warren Buffett, the world’s second-richest person, will give away 85% of his $44 billion fortune starting in July, a Berkshire Hathaway Inc. spokesman said yesterday, in the biggest individual pledge to charity ever.

Most of the money will go to the foundation started by his close friend Bill Gates, the only person in the world with a bigger personal fortune than Mr. Buffett, the 75-year-old Berkshire Hathaway chairman and chief executive officer.

“I know what I want to do and it makes sense to get going,” Mr. Buffett told Fortune in a story published online for the July 10 edition. The Berkshire chief financial officer, Marc Hamburg, confirmed the article.

Mr. Buffett’s commitment, based on the $3,071.09 closing price of Berkshire Hathaway Class B shares on June 23, is valued at more than $37 billion, exceeding the more than $29 billion that the chairman of Microsoft Corp., Mr. Gates, earmarked for his foundation.

“Warren Buffett is coming to terms with his mortality,” said Mohnish Pabrai, who manages $300 million at Pabrai Investment Funds, including $30 million in Berkshire shares.

Mr. Buffett plans to execute letters today, pledging annual gifts of company shares to the Bill & Melinda Gates Foundation; three organizations headed by his children, and the family foundation renamed in honor of his late wife, according to his company’s Web site.

“Bill Gates’s gift was always the largest; Buffett’s definitely tops that,” the editor of the Chronicle of Philanthropy in Washington, Stacy Palmer, said.

The Gateses and Mr. Buffett will discuss the pledges today at appearances in New York.

Mr. Buffett was traveling yesterday and unavailable for comment, Mr. Hamburg said. “You’ll have to ask Warren Buffett about that,” he said in a telephone interview.

Mr. Buffett has always maintained he wouldn’t give away his money until after his death, reasoning that he was the best person to increase its value, Ms. Palmer said. “This is a change of mind. He always said he was the best at making money and he could grow it.”

The death of his wife, Susan, in 2004 changed his mind, Mr. Buffett said in the interview with Fortune.

“Susie was two years younger than I, and women usually live longer than men,” Mr. Buffett said. “She and I always assumed that she would inherit my Berkshire stock and be the one who oversaw the distribution of our wealth to society, where both of us had always said it would go.”

More than 83% of the B shares will go to the $30 billion Gates foundation, the world’s largest philanthropic organization. “If your goal is to return the money to society by attacking truly major problems that don’t have a commensurate funding base,” Mr. Buffett told Fortune, “what could you find that’s better than turning to a couple of people who are young, who are ungodly bright, whose ideas have been proven, who already have shown an ability to scale it up and do it right?”

The Gates foundation thanked Mr. Buffett in a statement on its Web site.

“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 statement said. “The impact of Warren’s generosity will not be fully understood for decades.”

Mr. Buffett, in a letter to Mr. Gates, said the first year’s gift of 500,000 shares would increase the foundation’s annual giving by about $1.5 billion. The effect will be to more than double the $1.4 billion in grants given in 2005. In all,10 million shares are earmarked for the Gates foundation, which will receive 5% of the balance due annually, Mr. Buffett said.

“On an actuarial basis, Warren Buffett isn’t going to be around to get rid of half of it,” said Frank Betz, a Buffett bridge partner who helps manage $100 million at Warren, N.J.-based Carret Zane Capital Management, including Berkshire shares. “He’s doing the right thing at the right time.”

Mr. Buffett placed three conditions on the gift: Bill or Melinda Gates must be alive and active in the administration of the foundation; the organization must qualify as a charity, and the value of the gift must be spent in total, as well as at least 5% of the foundation’s net assets.

Mr. Buffett gave the foundation two years to abide by the third requirement.

Mr. Gates, 50, who competes in bridge tournaments with Mr. Buffett, will step down from his day-to-day role at Microsoft in July 2008 to commit more time to his foundation.

“Gates believes the money should be used while people are alive because there are so many needs,” Ms. Palmer said. “People have been watching to see what kind of influence Gates would have on Buffett. The timing of Gates’s announcement might have something to do with plans he and Buffett made.”

Mr. Buffett said in the Fortune interview that the events weren’t related.

“The timing is just happenstance,” he said, though, “I’m pleased that he’s going to be devoting more time to the foundation.”

While the number of shares donated will decrease over the years, the value should increase, he told Mr. Gates.

“This will make the foundation more ambitious,” Ms. Palmer said. “With this much money that must be given away fast and wisely, it will need Gates’s fulltime attention.”

The Gates foundation, which has made grant commitments since its inception of $10.5 billion, gave away $1.36 billion in 2005, according to its Web site. Major gifts and pledges included $1.5 billion to the Global Alliance for Vaccines, $1 billion to the United Negro College Fund, and $258 million to the Malaria Vaccine Initiative.

Mr. Buffett will give a total of 602,500 B shares to the five beneficiaries in July. Based on the June 23 closing price of $3,071.09, the first year’s gifts would be valued at $1.8 billion. The other recipients are the Susan Thompson Buffett Foundation, named for his late wife who died in 2004 at the age of 72 after a stroke; and those run by his children, the Howard G. Buffett Foundation; the Susan A. Buffett Foundation, and the NoVo Foundation, run by his son Peter.

Mr. Buffett, who owns almost 31% of the company, eventually will cut his holdings to 5%, which also will be given to a philanthropic endeavor.

According to letters posted on the Berkshire Hathaway Web site yesterday, Mr. Buffett earmarked 350,000 shares for each of his children, whom he addressed as Peter, Howie, and Sooz, to increase the grants of their foundations by more than $50 million in the first year. His late wife’s organization will receive 1 million shares, including 50,000 shares valued at $150 million in 2006.