Synchronously mitotic surface Plasmodia ofPhysarum polycephalum were ultra-violet-irradiated at different times during G2-phase (—4 h to —20 min with respect to metaphase), and treated immediately thereafter with varying concentrations of caffeine. It was observed that ultraviolet-induced mitotic delay is reduced significantly by this methylxanthine. In plasmodia irradiated between —4 and —1 h with respect to metaphase, the effect was concentration-dependent and the need for a certain threshold dose for obtaining the reduction in delay was apparent. However, higher doses than this were fairly toxic when applied at this part of the cycle and led to more mitotic delay than that obtained with UV alone. The most striking observation made during this study was the phase-specific precipitous effect seen in those plasmodia irradiated at about 20 min before mitosis which almost eliminated the long delay due to ultraviolet-irradiation. These results are discussed in the context of some of the known effects of ultraviolet and caffeine on a mitosis-promoting factor. It is proposed that the significant reduction of ultraviolet-induced mitotic delay reported here is due to the reactivation of the ultraviolet-inactivated mitosis-promoting factor by caffeine. Alternatively, it is possible that caffeine may prevent the inactivation of this factor by ultraviolet.