Genetic activity of trimethoprim in the Salmonella/microsomal screening system.

Trimethoprim, a widely used antibacterial drug was tested for its mutagenic potential in the Ames Salmonella/microsomal test system. The results indicated that, when used in the recommended dose range, the drug produced a several-fold increase in the reversion mutations on his(-)----his+ marker in some of the tester strains, compared with the spontaneous reversions. Dose-dependent curves were also obtained for reversion mutations caused by the drug. Ethyl methanesulfonate and benzo[alpha]pyrene were used a control mutagens.