The influence of dietary caffeine on the genotoxicity of the cooked food mutagen 2-amino-3,8-dimethylimidazo[4,5-f]-quinoxaline (MeIQx) was evaluated using the host-mediated assay in mice. For four weeks, BALB/c mice were fed a purified diet with or without caffeine (0.01% wt/wt in the diet). In the host-mediated assay, Salmonella typhimurium TA98 was given intravenously immediately before an oral dose of MeIQx (1.5 mg/kg body wt). After one hour, the mice were killed, the Salmonellae were recovered from the liver, and the number of mutants (his+ revertants) were determined. Consumption of caffeine led to a 47% reduction in the number of mutants induced by MeIQx (p < 0.001). Subsequent in vitro experiments using S. typhimurium TA98 revealed that the capacity of hepatic S-9 fractions from the caffeine-fed mice to covert MeIQx to an active mutagen was reduced by approximately 35%. This effect was not attributable to caffeine in the S-9 preparation. These data suggest that consumption of caffeine modifies MeIQx mutagenicity by altering the spectrum of enzymes involved in its activation.