Mutant lines of Chinese hamster ovary cells that show hypersensitivity to killing and mutagenesis by UV light were analyzed by genetic complementation analysis to determine whether defects in different gene loci might underlie a common cellular phenotype,. To facilitate rapid screening of mutant clones, a procedure was devised that allowed presumptive complementation to be assessed on the basis of the frequency of UV-resistant cells after fusion by polyethylene glycol. Four classes were identified among 44 clones tested. By using drug-resistance markers for selection of hybrid cells in crosses between UV mutant and wild type, a mutant from each of the four classes was shown to behave as phenotypically recessive. Hybrids were also isolated from crosses between each of the pair combinations of the four mutants. All such hybrids were relatively resistant to UV killing, providing confirmation of the complementation classes. When mutants representing the four UV-complementation classes were tested with the polyaromatic hydrocarbon 7-bromomethylbenz(a)anthracene, complementation was again seen for all pair combinations. These results suggest that each class of mutants represents a biochemical defect that plays a common role in the repair of both UV-induced and chemically induced lesions in the DNA.