[en] The induction by ultraviolet light of simian virus 40 (SV40) from two SV40 transformed hamster kidney cell lines is enhanced by caffeine. In order to investigate the mechanism responsible for this enhancement, the effect of caffeine on postreplication repair of DNA damaged by UV light was studied utilizing alkaline sucrose-gradient sedimentation. Caffeine at concentrations of 0.5, 1.0 or 2.0 mM inhibited the filling of gaps during postreplication repair. In addition, caffeine was found to potentiate cell killing by mitomycin C, an alkylating agent, and to enhance SV40 induction by mitomycin C. It is postulated that the persistence of gaps in DNA, caused by the presence of caffeine results in the enhancement of SV40 virus induction. (Auth.)