The mutagenic response to ultraviolet light and X-irradiation at a number of well characterized independent genetic loci, viz those conferring resistance to 6-thioguanine, ouabain, emetine, methylglyoxal (bis) guanylhydrazone (MGBG), and 5, 6-dichlororibofuranosyl benzimidazole (DRB), has been determined. Upon exposure of cells to UV light in the range of 70 to 180 ergs/mm2, the frequency of mutants at all of the above genetic loci increased in a linear dose-dependent manner. However, some genetic-locus-specific differences in response to UV light were observed in these studies. In contrast to UV light, exposure of cells to X-rays caused a linear dose-dependent increase in the frequency of mutants at only the thioguanine-resistant (Thgr) locus, which affects the purine salvage pathway enzyme hypoxanthine-guanine phosphoribosyltransferase, that is not essential for growth under these conditions. The other genetic loci that affect essential cellular functions (and hence should detect only specific base substitution types of mutations), however, showed no increase in the frequencies of mutants upon X-irradiation of cells. These results are in accordance with the nature of the genetic lesions which are caused by these agents and the properties of these genetic markers.