DNA repair after gamma radiation was studied in purified T lymphocytes from young and aged subjects. Two different assays were employed. In the first, T lymphocytes were stimulated with phytohemagglutinin (PHA) for 72 h and then treated with hydroxyurea, irradiated with 30 K rads and pulsed with [3H]thymidine (TdR) for 4 h. In the second, T lymphocytes were first irradiated with graded doses of gamma rays (200-800 rads) and then stimulated with PHA, cultured for 72 h and pulsed with 3H-TdR for the last 6 h of culture. T lymphocytes from aged subjects showed a lack of DNA repair synthesis in the first assay whereas only minor differences were found in the second assay between the two groups, i.e., a certain degree of radioresistance in aged lymphocytes. Lymphocyte superoxide dismutase activity showed great individual variations in both groups and a slight increase in old subjects.