Asynchronous and synchronous CHO cells were irradiated with germicidal UV light to determine the fluence response curve for cell killing, and the induction of resistance to 6-thioguanine, ouabain, and diphtheria toxin. For asynchronous populations the data show a sigmoidal response for induced reproductive death, as has been seen by others, with a D0 of 6 J/m2 and an extrapolation number of 2.5. The induction of mutations appears to be a linear function for all three mutagenic markers up to a dose of 17 J/m2. Reproductive death induced in the synchronous populations is a function of the time at which exposure occurs in the cell cycle, with late G1 and early S being the sensitive stages. The induction of resistance to 6TG, ouabain, and diphtheria toxin (DT) all seem to depend on the time of exposure in the cell cycle. As in the case of UV-induced reproductive death, the more sensitive periods for mutation induction appear also to be the G1 and early S period of the cell cycle, with the largest cyclic variation occurring for induced DT resistance. A comparison of the results reported here for the UV exposure with exposures of synchronous CHO cells to X-rays and ethylnitrosourea suggests that there are different age-specific responses to mutation induction for each agent, and that there are often different age responses for different mutagenic endpoints with the same mutagen.