Microsoft founder and multibillionaire Bill Gates announced today that his charitable foundation will give $750 million over the next five years to try to improve the health of young children in underdeveloped nations, his biggest annual philanthropic gesture yet.

The total amount of the immunization project is less than the $1 billion that Gates pledged three months ago to provide full college scholarships to academically qualified minority students in this country, but that endeavor is scheduled to last for the next 20 years.

Gates's announcement is the latest of several moves he has made recently to divert some of his enormous fortune--by all accounts he is the world's wealthiest person--to charity, something that some philanthropic groups have said he has been too slow to do. It also comes as his technology empire has been under fire in the courts.

Earlier this month, a federal judge in Washington issued a preliminary ruling that Microsoft Corp. is a monopoly in the computer software industry and is unfairly squelching competition and harming consumers with its business tactics. But U.S. District Judge Thomas Penfield Jackson has not ruled on whether the company violated federal antitrust laws and is not expected to do so until next year. Jackson is encouraging the Justice Department and Microsoft to reach a negotiated settlement.

Gates and his wife, Melinda, have often expressed interest in aiding the cause of children's health and already have given more than $100 million to groups working on that issue in poor countries. The latest gift dwarfs all previous ones and will be focused on expanding immunization programs around the world.

By some public health organization estimates, about 4 million children die each year of illnesses that could be prevented if they were vaccinated in time. And in some Third World countries it can take a decade or more for vaccines to reach children who need them the most. Officials of the foundation said their goal is to double the number of children saved from disease each year. But they had no estimate yet of how many children that would be.

"Melinda and I are committed to ensuring that every child has access to lifesaving vaccines in the next millennium, regardless of where they live," Gates said in a statement released by his foundation late today. It had been planning to make an announcement later this year, but word of the pledge apparently leaked.

Gates said he expects the project to be underway and purchasing and distributing vaccines to children in about eight months. A new coalition called the Global Fund for Children's Vaccines will be formed as part of the plan.

Its partners will include the World Health Organization, the United Nations Children's Fund (UNICEF) and the World Bank. The groups have been working on the collaboration for nearly a year, officials of the foundation said. They also said they expect to work closely with governments in an attempt to ensure the program does not get mired in politics or bureaucracy.

Carol Bellamy, the executive director of UNICEF, praised the undertaking. "Today's commitment by Bill and Melinda Gates is a major step forward in ensuring that all children will have equal access to life-saving vaccines," she said in a statement.

James Wolfensohn, president of the World Bank, called Gates's gift a "tremendous boost" to one of the most difficult goals in public health.

Officials with the Bill and Melinda Gates Foundation said that they expect philanthropies and private donors to make other large donations to the project. They said more details about the $750 million immunization fund will be made public early next year at the World Economic Forum in Davos, Switzerland.

The charitable foundation run by Gates and his wife is the largest in the country, with more than $17 billion in assets. Gates's personal fortune, most of it in Microsoft stock, exceeds $100 billion.

His recent pledge to spend $50 million a year for 20 years on college scholarships was among the largest donations in the nation's history and by far the largest in education. As part of it, every year 1,000 talented but financially needy minority students will be able to attend a university of their choice, at no cost. Gates has said one purpose of that program is to get more minorities involved in sciences and technology.

Gates is also financing Internet connections for libraries and needy schools around the country, but some have called that gesture a bit self-serving to his computer software empire.