Caffeine significantly repressed MMC induced reversion of Salmonella typhimurium strain TA92 from his- to his+. The addition of MMC (0.5 micrograms/ml) to broth cultures resulted in a slightly decreased growth rate, as measured turbidimetrically, along with a linear decline in CFU of 1 log cycle in 7 h. The addition of caffeine (0.33 mg/ml) plus MMC (0.5 micrograms/ml) resulted in cessation of growth and a decrease in culture turbidity after two generations of growth which was accompanied by a linear decline in CFU of four log cycles in 7 h. The repression of MMC induced mutagenesis by caffeine is therefore most probably due to interference with an error-prone repair process which results in the destruction of cells with damaged DNA.