Using synchronous populations obtained by selectively detaching mitotic cells from cultures grown in monolayer, we demonstrate here that Chinese hamster ovary (CHO) cells exhibit a differential sensitivity to mutation induction by UV as a function of position in the cell cycle. When mutation induction to 6-thioguanine (TG) resistance is monitored, several maxima and minima are displayed during cell-cycle traverse, with a major maximum occurring in early S phase. Although cells in S phase are more sensitive to UV-mediated cell lethality than those in G1 or G2/M phases, there is not a strict correlation with induced mutation frequency. Fluence-response curves obtained at several times during the cell cycle yield Dq values approximating 6 J/m2. The primary survival characteristic which varies with cell cycle position is D0, ranging from 2.5 J/m2 at 6 h after mitotic selection to 5.5 J/m2 at 11 h afterward. Based on studies with asynchronous, logarithmically growing populations, as well as those mitotically selected to be synchronous, the optimum phenotypic expression time for induced TG resistance is 7--9 days and is essentially independent of both UV fluence and position in the cell cycle. All isolated mutants have altered hypozanthine-guanine phosphoribosyl transferase (HGPRT) activity, and no difference in the residual level of activity was detected among isolated clones receiving UV radiation during G1, S, or late S/G2 phases of the cell cycle. Changes in cellular morphology during cell-cycle traverse do not contribute to the differential susceptibility to UV-induced mutagenesis.