Ade-C is a Chinese hamster ovary cell line auxotrophic for purines because of a mutation in the de novo synthetic pathway. We now show that, in the absence of exogenous hypoxanthine, replicative DNA synthesis is rapidly shut down. Various aspects of DNA repair have been studied in purine-starved cells. Incision, the first step of excision repair of UV damage, appears normal, as do the later steps, repair synthesis (demonstrated following chemical damage as well as UV-irradiation) and ligation. However, removal of UV-induced pyrimidine dimers is not detected, and it seems that the repair that occurs is aberrant. This behaviour is associated with an increase in cell killing by UV light, and a several-fold increase in the frequency of mutations induced by UV.