Lewis Hughes, 51, the director of the office of international activities at the Environmental Protection Agency, died Friday at George Washington University Hospital. He had lymphoma.

Dr. Hughes joined the EPA in 1978 after several years as a health and safety administrator at National Aeronautics and Space Administration, the University of California at Berkeley and the old Atomic Energy Commission. He was born in Hilltop, W.Va., earned a bachelor's degree in biology and physics at West Virginia State College in 1951 and a master's degree in biophysics and physiology at West Virginia University in 1956. He received a doctorate in environmental health sciences from the University of California at Berkeley in 1972.

In 1956, Dr. Hughes moved to Livermore, Calif., where he was a health physicist at the Lawrence Livermore Laboratory, a facility of the old AEC. He remained there until 1965, when he became radiological safety officer at the University of California at Berkeley. It was during his years at Berkeley that he earned his Ph.D. He also was a lecturer at the School of Public Health at Berkeley.

From 1972 to 1978, Dr. Hughes was chief of the health and safety office at the Ames Research Center, a NASA facility, and also worked with members of the European Space Agency. He received NASA's exceptional service award in 1976 and its group achievement award in 1977.

He moved to the Washington area in 1978 and was named deputy associate administrator for international activities of the EPA. He became acting director last September and was named director of the office shortly before his death.

He belonged to several professional organizations and published more than 30 papers in professional journals.

Dr. Hughes was a lieutenant in the Army from 1951 to 1953.

Survivors include his wife, the former Constance Pindle, and two children, David and Dana, all of Bethesda, where Dr. Hughes resided; a sister, Elizabeth H. Patterson, of Hilltop, W.Va., and two brothers, James, of Columbus, Ohio and Robert, of Castro Valley, Calif.