The homogenates of 16 foodstuffs were dried and extracted with methanol using Soxhlet apparatus. These methanol extracts were subjected to mutagenicity assay against Salmonella typhimurium TA 100 with or without S9 mix by the preincubation method. It was found that meat, fish and egg induced large numbers of His+ revertant colonies with and without S9 mix. Studies were also carried out to test the changes of the numbers of revertant colonies caused by nitrite treatment of foodstuffs. Most of the foodstuffs showed particuarly high incereases of His+ revertant colonies after nitrite treatent. Much larger increases were obsereved in heated foodstuffs than in raw foodstaffs. The increase of mutagenic activities was generally small in the presene of S9 mix. Further, effect of nitrite treatment on three different style foods was also examined. The increase of mutagenic activity observed in Japanese-style food was much smaller than those observed in others after nitrite treatment.