We previously reported that smoke condensates from fish showed high mutagenic activity (Nagao et al., 1977a ; Sugimura et al.,1977) . Charred material in the surface layer of broiled fish and meat also contained a high mutagenic potential (Nagao et al., 1977a; Sugimura et al., 1977). Subsequently it was revealed that mutagenic principles were formed by pyrolysis of protein but not carbohydrate, nucleic acid and vegetable oil (Sugimura et al., 1977). This report deals with mutagenic principles in the pyrolysis products of D, L-tryptophan and D, L-phenylalanine. The most plausible structures from the former were 3-amino-1, 4-dimethyl-5H-pyrido [4, 3-b] indole and 3amino-l-methyl-5H-pyrido [4, 3-b] indole. The structure of the active principle of D, L-phenylalanine was proved to be 2-amino-5-phenylpyridine. Since pyrolysis products are very likely to be produced by cooking proteinous food, such as fish and meat, by direct exposure to the flame, and since the correlation between mutagens and carcinogens have been generally accepted in recent investigations (McCann et al., 1975 ; Sugimura et al., 1976), there is a strong probability that cooked foods in which charred material from proteinous components are produced might play a role in carcinogenesis generally occurring in human. Mutagenicity tests were carried out using an improved method (Nagao et al., 1977b; Yahagi et al., 1977) of the Ames test (Ames et al., 1975). Test material, bacteria and S-9 Mix prepared from the liver of rats treated with polychlorinated biphenyl were prein-