The Bill & Melinda Gates foundation is giving a $500million grant to an international fund that provides Aids assistance in poor countries.
The Geneva-based global fund to fight Aids, tuberculosis and malaria will receive the grant over five years.
The Gates charity has already given the global fund $150million since it was created four years ago and announced a donation of $287million to help speed up the development of an Aids vaccine last month.
In a statement on Wednesday, Bill Gates said the fund "is one of the most important health initiatives in the world today".
The announcement comes as nearly 25,000 scientists, advocates and policy-makers prepare to meet in Toronto this weekend for the 16th international Aids conference.
Governments have been the main contributors to the global fund, whose total income to date is $9billion. Fund organisers say that this figure falls far short of what is needed and that the level of support from rich nations, especially the United States, has been controversial.
Joanne Carter, of an advocacy organisation called Results, said in a statement representing a coalition of such groups: "When the richest man on earth provides such generous support for the global fund, the risk is that some donor governments may mistakenly think they are now off the hook."
However, Melinda Gates, co-chair of the Gates foundation, said she hoped the grant would encourage more donations not less: "We hope all donors - public and private, large and small - will step up their support and make long-term commitments."
So far, 132 countries have received money from the global fund. About 544,000 people have received treatment for Aids and more than 1.4million for tuberculosis.