The relationship of the rate of epidermal proliferation to ultraviolet (UV)-induced cell death, namely sunburn cell (SC) formation, was investigated in vivo using the guinea-pig model. The number of SCs was correlated with (1) cellophane tape stripping which stimulates the proliferation rate and (2) treatment with antimitotic and antimetabolic agents which suppress the proliferation rate. The tape stripping performed 14 h before UV irradiation significantly increased the number of SCs counted 24 h after UV irradiation from a mean of 17.4 to 62.6 per section, and the tritiated thymidine (TdR-3H)-labelling index (LI) increased from a mean of 8.1 to 27.4%. SC counts and LI were markedly decreased by the intradermal injection of colchicine, methotrexate, hydroxyurea or 5-fluorouracil given once immediately after the tape stripping. The autoradiographic study showed that 23.2% of SCs were labelled when TdR-3H was given intradermally 1 h before UV exposure, while no SCs were labelled when TdR-3H was given immediately after UV exposure. The labelled SC count (23.2%) which presumably represents S phase cells at the time of UV exposure, was higher than the LI (mean 8.1%). These results suggest that proliferating cells, possibly S phase cells, may be responsible for SC formation.