The Howard Hughes Medical Institute in Bethesda, planning for a bright future after the $5 billion addition to its endowment last month, is beefing up its management team.

HHMI, the country's largest private medical research organization, which nonetheless has maintained a low profile over the years, was the beneficiary of a $5 billion windfall from the sale of its principal asset, Hughes Aircraft Co., to General Motors Corp. in June. The sale made HHMI the largest charitable organization in the country and probably the world, easily surpassing the Ford Foundation's $3.6 billion in assets, observers said at the time of the sale.

In the first indication of how income from the huge sum might be spent, HHMI has created a second management tier, which will report to President and Chief Executive Officer Donald S. Fredrickson. It will consist of five men "at the top of their respective disciplines," said Fredrickson, a former director of the National Institutes of Health.

The appointments, announced Thursday, are:

*Purnell W. Choppin, 56, vice president and chief scientific officer. Choppin will direct the institute's core laboratory program -- a joint university-institute venture -- design and select new programs, and oversee the operations of 17 laboratories and more than 200 scientists. Until the appointment, he was professor of virology, vice president for academic programs and dean of graduate studies at Rockefeller University in New York.

*George Francis Cahill, 58, vice president for scientific training and development. A Harvard Medical School professor and former director of Joslin Research Laboratories, Cahill will supervise the recruitment, training and development of the institute's research scientists.

*David J. Taylor, 41, vice president, general counsel and chief administrative officer. He will direct administrative and personnel matters, including a plan to move the institute's headquarters to a central location in the Washington area. HHMI currently maintains administrative offices in Coconut Grove, Fla., in addition to its executive offices in Bethesda. Taylor is a general partner specializing in corporate law in the firm of Schiff, Hardin & Waite and a former U.S. Court of Appeals law clerk. His immediate legal responsibilities will include closing the sale of Hughes Aircraft and settling the institute's tax cases.

*Robert C. White, 65, vice president and chief financial officer. He was assistant treasurer of the Ford Motor Co., where he had responsibility for Ford's $11.5 billion in pension and trust investment assets. As controller, he will direct the institute's internal financial operations.

*Graham O. Harrison, 62, vice president and chief investment officer. Formerly president of the U.S. Steel and Carnegie Pension Fund -- which manages employe benefit assets of $7 billion -- Harrison will manage the institute's investments.

The institute, which sponsors medical research in the fields of genetics, immunology, metabolic control and neuroscience, announced it will begin increasing its disbursements for this year and beyond. But there are no details about how the investment income will be spent, according to Taylor, the new chief administrative officer.

A detailed analysis is being conducted by the institute's board and may be made public after the board's meeting in October, Taylor said. He added that although the sale -- expected to be completed in September -- will lead to additional revenue and additional funding for current programs, the focus of HHMI's research will not change.

HHMI was founded by aviation industrialist Howard R. Hughes in 1953. Under terms of a court order following his death in 1976, nine trustees were named to manage the institute.