The effect of caffeine on nitrosoguanidine-induced mutagenesis of Salmonella typhimurium and its P22 and L phages was studied. The detected mutations included phage "clear" mutations, reversions of phage "amber" mutation, and prototrophic reversions of the his- auxotroph of Salmonella typhimurium. Neither the recA mutation of the host nor the erf mutation of the phage genome were found to affect the nitrosoguanidine-induced mutagenesis of the phage during vegetative growth. Beginning with a concentration of 0.2 mg/ml, caffeine decreased the frequency of mutants by 30--60%, attaining a maximum effect at 1.5 mg/ml and retaining this effect even at higher concentrations. A similar antimutagenic effect was observed with the mutagenesis of the host cells. The nitrosoguanidine-induced mutagenesis does not seem to be related to the function of the recA cell gene or the erf phage gene. The mechanism of mutagenesis by nitrosoguanidine probably has two components, one of them caffeine sensitive, the other caffeine-resistant.