The inhibitory potential of various food constituents on the mutagenicity generated in fava beans after treatment with nitrite has been investigated. Cheese was able to inhibit this direct-acting mutagenicity. The antimutagenic factor was not extractable from cheese; solvents of different polarity were used for the extraction. Casein, pectin, gelatin, Vicia faba protein and, to a lesser extent, whey protein and starch could also inhibit mutagenicity. A decrease in mutagenicity was always accompanied by a decrease in total N-nitroso content, as measured analytically. The mutagenic principles appeared to bind more strongly onto cheese than onto V. faba. The implications of this finding are discussed.