In vitro antimutagenicity of Bacillus natto islolated from Natto, Japanese traditional fermented food, was investigated using umu-test. Mutagenicity of S9-activated metabolites of Trp-P-2 and IQ for Salmonella typhimurium TA 1535/pSK1002 was remarkably inhibited by addition of bacterial cells and their cytoplasmic fraction. Desmutagenicity by cytoplasmic fraction increased with increasing concentration of the fraction. Bioantimutagenic effect of cytoplasm on Salmonella typhimurium SD-100 did not show bioantimutagenic activity against mutated bacterial cells induced by Trp-P-2. Cytoplasmic fraction exhibited 17% bioantimutagenicity due to desmutation caused by IQ.