We have previously reported the isolation and partial characterization of DNA repair and/or mutagen-sensitive mutant Chinese hamster cell strains. Here we present the results of a detailed study of the ultraviolet light (UV)-induced mutability of one of these strains, UVs-7, and provide preliminary mutability data on two additional lines, UVr-23 and UVs-40. UVs-7 in extremely deficient in unscheduled DNA synthesis (UDS) but only slightly more sensitive to UV than the parental line. When examined for the UV-inducibility of mutants resistant to ouabain, 6-thioguanine, or diphtheria toxin, UVs-7 was found to be hypermutable at all three loci as compared to the parental line. The degree of hypermutability was not the same for any two loci. UVs-40, a highly UV-sensitive strain, was also found to be hypermutable at the ouabain-resistant (ouar) locus. UVr-23, which is UV-resistant and more proficient at UDS than the parental line, appeared to exhibit a tendency toward hypomutability at both the ouabain(ouar) and 6-thioguanine--resistant (6TGr) loci. Further characterization of all these lines should aid in delineating mammalian mechanisms of DNA repair and mutagenesis.