The rate of DNA-chain elongation was studied in mouse FM3A cells after treatment with 8-methoxypsoralen plus near-ultraviolet radiation using the minimal doses (1 microgram/ml 8-methoxypsoralen plus 1-2.5 kJ/m2 of near-ultraviolet radiation) which inhibited cell-cycle progression or DNA replication. A rapid decrease in incorporation of [3H]thymidine and recovery to some extent during incubation after treatment have been reported (Hyodo, M., Fujita, H., Suzuki, K., Yoshino, K., Matsuo, I. and Ohkido, M. (1982) Mutat. Res. 94, 199-211). The results of the present study showed that the rate was not changed suggesting that the decrease in [3H]thymidine incorporation was not due to the rate of DNA-chain elongation, but was due to change in the frequency of initiation of replication. Formation of DNA crosslinks was then studied by the sedimentation of pre-labeled DNA in an alkaline sucrose gradient. The results showed that, at these doses of 8-methoxypsoralen plus near-ultraviolet radiation, approx. 2-7 crosslinks were formed per 10(9) Da. It was also suggested that some of the DNA crosslinks might be repaired during the prolonged incubation, but unrepaired crosslinks were still present after 24 h incubation.