Identification of mutagenic factors in foods is of concern because they may represent carcinogens to man. Cooked foods, especially their basic fractions containing heterocyclic amines, have high mutagenicity; the neutral fractions containing mutagenic nitropyrenes, however, have not been studied in detail. The mutagenicity of various grilled foods--10 vegetables, 4 fish, and 4 kinds of meat with and without sauce--was studied. The concentration of 1-nitropyrene was measured after reduction by a specific nitroreductase purified from Bacteroides fragilis. 1-Nitropyrene was detected in grilled corn, horse-mackerel, and mackerel, and accounted for less than 10% of the total mutagenicity of the crude extracts in the Salmonella mutation test using strain TA98 in the absence of S9 mix. The mutagenicity of the diethyl ether-soluble basic fractions of meat grilled without a marinating sauce was very high. However, the sauce decreased the mutagenicity of the basic fractions and increased the mutagenicity of the neutral fractions. Moreover, considerable amounts of 1-nitropyrene were detected in pork and yakitori (grilled chicken) grilled with the sauce. The neutral fractions of yakitori grilled for 3, 5, and 7 min contained 3.8, 19, and 43 ng, respectively, of 1-nitropyrene per gram of yakitori, accounting for 3.0, 2.7, and 1.3%, respectively, of the total extract mutagenicity. We conclude that formation of 1-nitropyrene in the yakitori is due to pyrene produced by incomplete combustion of fat in the chicken, its nitration at acidic pH by nitrogen dioxide emitted by burning of cooking gas, and some components of the marinating sauce. Antimutagenic activity of edible mushrooms against 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indol (Trp-P-1) was also studied.