Soya sauce showed marked direct-acting mutagenicity toward Salmonella typhimurium TA 100 after nitrite treatment. Three precursors showing mutagenicity after nitrite treatment were isolated from soya sauce. Their structures were determined to be (-)-(1S,3S)-1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid [(-)-(1S,3S)-MTCA], its stereoisomer (-)-(1R,3S)-MTCA and tyramine. The numbers of revertants of TA 100 induced by 1 mg each of (-)-(1S,3S)-MTCA, (-)-(1R,3S)-MTCA and tyramine, after nitrite treatment, were 17 400, 13 000 and 3 900, respectively, without S9 mix. The amounts of MTCA isomers and tyramine in various Japanese soya sauces showing mutagenicity after nitrite treatment were 82-678 and 17-2 250 micrograms/mL, respectively. Most soya sauces produced in the USA showed weaker mutagenicity than those produced in Japan and contained lower, if not undetectable, amounts of the three precursors of mutagens. The mutagenicity of MTCA isomers and tyramine accounted for 16-61 and 1-35%, respectively, of the mutagenicity of the soya sauces after nitrite treatment. The mutagen(s) produced from (-)-(1S,3S)-MTCA with nitrite was a minor product(s), the major product being the non-mutagen, (-)-(1S,3S)-1-methyl-2-nitroso-1,2,3,4-tetrahydro-beta-carboline-3-carbo xylic acid [(-)-(1S,3S)-MNTCA], but the mutagen 4-(2-aminoethyl)-6-diazo-2,4-cyclohexadienone, produced from tyramine with nitrite, was one of the major products.