Two years of sequential studies of the UVR (254 nm) response in normal human lymphocytes showed pronounced seasonal variations. In the winters of 1981-82 and 1982-83 the UVR-induced DNA synthesis in the lymphocytes in vitro (the DNA-repair synthesis) was very stable with inter- and intra-individual variations below 14 and 10%, respectively (15 individuals). In the summers 1982 and 1983 the inter-individual variations was 2-3-fold higher. Closer analysis revealed that this was due to a pronounced intra-individual variation with sharp fluctuations in the single individual. Also the UVR tolerance of the lymphocytes estimated by alterations of the phytohaemagglutinin (PHA) stimulated lymphocyte proliferation showed seasonal variations, being 1.5-fold higher in the winter than in the summer (P less than 0.0001, 12 individuals). The observed seasonal variations emphasize the importance of considering chronobiological variations when working with long term projects. The UVR-induced DNA synthesis was determined as the increase in incorporation of [3H]-thymidine in irradiated cells compared to non-irradiated cells after incubation for 2 h at 37 degrees C in phosphate buffered saline. The UVR tolerance was measured by the UV dose necessary for 50% reduction in PHA stimulated lymphocyte proliferation assessed after 7 days of growth.