Dr. James C. Cain, a specialist in internal medicine at the Mayo Clinic for 30 years and the longtime personal physician to President Lyndon B. Johnson and his family, died on Saturday at his in Rochester, Minn. He was 78 years old.

     Dr. Cain died of cancer, said Hoyt J. Finnamore, a spokesman for the Mayo Clinic.

     Dr. Cain gained national prominence in October 1965 when he was attending physician for President Johnson's gall bladder surgery. Also at the President's side in November 1966, Dr. Cain said he was "perturbed" over his patient's rapid pace of activities after surgery on the throat and the abdomen.

     President Johnson appointed Dr. Cain chairman of the National Advisory Committee on Selective Service in 1965, a position he held until 1968. Dr. Cain also served as a member of the National Advisory Heart Council from 1962 to 1966.

     When he retired in 1978, Dr. Cain was a member of the Mayo Clinic's Department of Internal Medicine and professor of medicine at the Mayo Medical School. He served as professor of clinical medicine at the Mayo Graduate School of Medicine from 1964 to 1973, and section head in the Mayo Clinic Division of Gastroenterology and Internal Medicine from 1967 to 1970.

     Born in Kosse, Tex., Dr. Cain graduated from the University of Texas, where he received his M.D. degree in 1937.

     In October 1940, he entered the Mayo Graduate School of Medicine in Rochester as a fellow in internal medicine. The following year, he joined the Army Medical Corps, serving in Europe and the United States, attaining the rank of colonel.

     Dr. Cain was president of the Minnesota State Board of Medical Examiners in 1971 and 1972. He received the Gold Medal of the American Medical Association in 1963.

     He is survived by his wife, Ida May of Rochester; three daughters, Stephanie Van D'Elden and Katherine May Snider, both of Minneapolis, and Mary Lucinda Moore of Livonia, Mich.; a son, James Cain of Missoula, Mont.; a brother, Thomas Cain of Dallas, and nine grandchildren.