Magaret Green McElwain, 63, a science writer for the National Institutes of Health who was active in hospital and church work, died Friday at the NIH Clinical Center where she was being treated for cancer.

Mrs. McElwain joined the NIH information office in 1960. She was a senior member of the office of information in the Institute of Allergy and Infectious Diseases for 10 years. In 1973 she became special assistant to the institute director.

In 1977, she received the NIH Merit Award "for both her writing ability and her management of research reporting efforts." She twice received the Blue Pencil Award from the Federal Editiors Association. In 1975, she received an award of merit from the Washington chapter of the Society for Technical Communications.

In a tribute, Dr. Richard M. Krause, director of the Institute of Allergy and Infectious Desease, said: "Magaret had that rare ability to grasp difficult concepts of medical science and translate them with style and ease so that all could perceive their intricate beauty."

Mrs. McElwain was born in Nashville, Tenn. She earned a bachelor's degree in journalism from Northwestern University.

A resident of Westmoreland Hills, Md., she came to this area in 1948 after her marriage to Washington attorney Edwin McElwain. He died in 1960.

She was a past president of the Hospital for Sick Children in Washington and a member of the National Association of Science Writers.

Mrs. McElwain was a member of Westmoreland Congregational Church and was secretary of the board of Christian education at her death. She also had taught church school at Metropolitan Memorial Methodist Church.

Survivors include three sons, Charles e., of New Orleans, James r., of Salt Lake City, Utah, and William g., of Washington, and her mother, Mrs. J. Ross Greene, and a sister, Mrs. John Peebles, both of Nashville.

The family suggests that expressions of sympathy be in the form of contribution to the American Cancer Society or to The Hospital for Sick Children. CAPTION: Picture, MARGARET G. McELWAIN