Induction of mutations to 6-thioguanine resistance (TGr) by gamma-rays at three different dose-rates and molecular changes in the HPRT gene were studied in human lymphoblastoid WIL2-NS cells. Mutant induction showed a curvilinear dose-response for acute irradiation (30 Gy/h). The total mutant frequency was lower after irradiation at 0.17 or 0.006 Gy/h compared with acute irradiation. An apparent linear relationship between total dose and mutant frequency was found for the chronic irradiations. Spontaneous mutant frequency increased linearly with the exposure time of protracted irradiation at 0.006 Gy/h. After the spontaneous mutant frequency was subtracted from the total mutant frequency for irradiation at 0.006 Gy/h, no significant difference was found in the mutant frequency as a function of dose between the cultures irradiated at 0.17 Gy/h and those at 0.006 Gy/h. The inverse dose-rate effect, which has been observed in proliferating mouse L5178Y leukemia cells was not evident in WIL2-NS cells at the dose-rates employed. Structural alterations at the HPRT locus in TGr mutants were examined with the multiplex PCR method and compared among cultures irradiated at different dose-rates. Assuming that the mutants isolated were primarily independent, approximately 17% of spontaneous mutants were deletion mutants. When the fraction of spontaneous mutants in the irradiated cultures was subtracted from the total fraction of each type of mutant, it is clear that low dose-rate gamma-rays induced deletion mutations at the HPRT locus just as efficiently (79%) as high dose-rate gamma-rays (74%).