Ultraviolet irradiation (254 nm) of immortal diploid human lymphoblasts killed cells, caused mutation at three genetic loci studied, and transiently inhibited 3H-TdR uptake into DNA. A shoulder of about 6 J/m2 and a D0 of 6 J/m2 was observed for survival. Mutation rose in a monotonic non-linear fashion through 6 J/m2; above 6 J/m2, complex behavior approximating a plateau in induced mutation was observed. Irradiation at 4.4 J/m2 caused a transient increase in the number of cells synthesizing DNA and a decrease in the rate of DNA synthesis relative to mock-irradiated controls. The parameter of rate of DNA synthesis per cell in DNA synthetic phase showed a rapid recovery toward control values between 2 and 4 h after irradiation and a slower recovery to control values by 22 h post-irradiation. Fractionated dose schedules were used to measure the effects of allowing a time interval between doses at nontoxic fluences (2.2 J/m2), moderately toxic fluences (8.8 J/m2) and toxic fluences (17.6 J/m2). These measurements indicate that in the non-toxic range of fluences common to human exposure, mutational response is mediated by a post-irradiation process which seems to show an enhanced ability to protect against mutation induced by subsequent irradiation. However, at moderately toxic fluences there was little effect of dose fractionation, and at toxic fluences, a time-dependent increase in mutation fraction was observed at separation times greater than 7 h. We suggest that these latter observations arise primarily from cell cycle heterogeneity with regard to sensitivity to UV killing and mutation.