[en] The effect of prior incubation with [³H]thymidine on survival and mutagenesis after X-irradiation of human lymphocytes was studied by incubating lymphocytes with 0.001-1.0 μCi/ml [³H]thymidine for 6 h at 37⁰C and then irradiating with 150 or 300 rad. Survival was measured using lymphocyte cloning and mutagenesis was measured using 6-thioguanine selection to detect clones mutated at the hypoxanthine phosphoribosyltransferase locus. [³H]Thymidine alone had no effect on survival or mutagenesis and X-radiation alone produced the expected decrease in survival and increase in mutations. [³H]Thymidine prior to X-radiation had no effect on lethality of X-radiation but at concentrations of 0.1 and 1.0 μCi/ml produced a significant decrease in the number of mutations induced after both 150 and 300 rad. The results suggest that ionizing radiation, produced by disintegration of ³H, reduces the mutagenic effect of a subsequent exposure to ionizing radiation by induction of a system which prevents of repairs a restricted class of radiation damage. (Auth.)