The suppressive effects on SOS response of Salmonella typhimurium TA 1535/pSK 1002 induced by MNNG and Trp-P-1 of the methanol extracts prepared from soybeans and soy products were investigated. The content of isoflavones in the extract was also determined. The extracts of soybean and soymilk scarcely suppressed SOS response by mutagens. The suppression by the extracts of fermented product was stronger than that by the unfermented products. However, the percentage suppression in the extracts of natto and soy sauce was similar to that in the unfermented products. The extracts of tempeh and miso having strong suppression contained much daidzein and genistein. On the other hand, the extracts of unfermented product and such fermented product as natto and soy sauce having weak suppression contained much isoflavone glycoside. The percentage suppression of the extracts from soymilks fermented by a strain of Lactobacillus bulgaricus or Streptococcus thermophilus was lower than that by a strain of Bifidobacterium longum or a mixture of three strains.