American business giant Warren Buffet has just donated billions of dollars to charity, as part of his 10-year plan to give away his entire fortune.

The 86-year-old boss of Berkshire Hathaway, who is the world's fourth richest person, transferred US$3.17 billion ($4.4b) of stock to various not-for-profit organisations, marking his largest-ever philanthropic contribution.

The transfer is Buffett's 12th annual donation to the five charities and was made up of more than 18 million shares in his huge company.

The biggest beneficiary of the unprecedented gift was the Bill and Melinda Gates Foundation, as well as four family charities.

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The Gates Foundation, which focuses on improving health, reducing poverty and aiding education, is receiving about US$2.42 billion of Monday's donations.

Buffett also donated to the Susan Thompson Buffett Foundation, named for his late first wife, and the Howard G. Buffett, Sherwood and NoVo Foundations, respectively overseen by his children, Howard, Susan and Peter.

The transfer is Buffett's 12th annual donation to the five charities. He has given away more than US$27.5b since 2006.

Things took on new momentum in 2010 when he launched The Giving Pledge with billionaire mate Gates, encouraging the richest people in the world to dedicate the majority of their wealth to good causes.

As of last month, some 170 people had signed up.

Despite handing over a big chunk of change overnight, Buffet hasn't shifted from his fourth place on the Forbes rich list.

Before the donations were announced, Forbes estimated Buffett's net worth at US$76.3b, trailing Microsoft Corp co-founder Bill Gates' US$89.4b, Spanish retailing magnate Amancio Ortega's $US81.8b and Amazon.com Inc founder Jeff Bezos' US$84.8b.

Gates is a close friend of Buffett and a Berkshire director. Buffett typically reduces the number of shares he donates by five per cent from the prior year.

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The charities usually sell the Berkshire shares to finance their activities, reflecting Buffett's desire that his money be spent. Buffett also makes smaller donations to other charities.

- news.com.au