The world's second-richest person, Warren Buffett, is donating $37 billion - more than 80% of his fortune - to the charitable foundation run by his friend Bill Gates.
The move is the biggest-ever single act of charitable giving in the US.
Mr Buffett will hand 10 million shares in his Berkshire Hathaway firm to the Bill and Melinda Gates Foundation.
The announcement from Buffett, 75, comes just days after Microsoft Chairman Gates said he would move away from his day-to-day role at the software giant to focus more on charity work.
The Gates foundation is one of the world's richest philanthropic organisations and aims to fight disease and promote education around the world.
Buffett, chief executive of Berkshire Hathaway, is worth an estimated $44 billion, according to Forbes magazine.
That makes him the world's second-richest man behind Gates, who is worth about $50 billion.