Following exposure to ultraviolet irradiation (UV), two of three human fibroblast strains and one of three melanoma cell lines showed lower rates of thymine dimer excision during 24 h at 40 degrees C than at 36 degrees C. All lines had lower rates at 32 degrees C. Autoradiographic studies of three fibroblast strains and four melanoma lines incubated for four hours after irradiation revealed decreased unscheduled DNA synthesis at 42 degrees C compared with 36 degrees C. The rate of semiconservative DNA synthesis was decreased at the upper temperature in both series of experiments. All eight cell lines tested showed decreased repair at 42 degrees C, as judged by slower sedimentation and increased heterogeneity of parental DNA in alkaline sucrose gradients. Experiments using the DNA synthesis inhibitor Actinomycin D suggested that these effects were due to temperature-sensitive repair synthesis. In the two lines studied, preincubation of cells at 42 degrees C apparently increased the extent of UV damage. Although by no means conclusive, these results are consistent with the possibility that temperature-sensitive DNA repair is a contributory factor in some cases of solar carcinogenesis.