Earlier experiments on human cells showed that N-acetoxy-2-acetylaminofluorene mimics ultraviolet radiation in biological and repair characteristics and that the amount of repair from a combined treatment was additive. Chinese hamster V-79 cells are less proficient than human cells in excision repair of pyrimidine dimers resulting from irradiation. We therefore investigated the combined effects of both agents on repair in V-79 cells to see whether they follow the same pattern as in human cells. They did not. Measurements of unscheduled DNA synthesis and the photolysis of DNA repaired in the presence of bromodeoxyuridine gave information about repair due to both agents, and the use of an endonuclease in an extract of Micrococcus luteus allowed us to measure repair of only ultraviolet damage in the presence of N-acetoxy-2-acetylaminofluorene damage. Each technique indicated that the amount of repair from a combined treatment was less than additive and in some cases less than that due to either agent. We conclude that V-79 cells are different from human fibroblasts in the excision repair of both ultraviolet and N-acetoxy-2-acetylaminofluorene damage and suggest that both kinds of damages inhibit repair of damage due to the other agent.