This study evaluated the possibility that extracts from some edible mushrooms (button, shiitake, oyster and abalone) could modulate the formation of the mutagen precursor during extended period cooking of beef and the direct mutagen produced from interaction between nitrite and beef concentrate. The concentrates from beef boiled with the mushroom samples did not show the mutagenicity when tested with the Ames test using Salmonella typhimurium strains TA98 and TA100 in the absence of metabolic activation. After treated with nitrite, they exhibited the mutagenicity towards both strains. The results showed that the addition of mushroom extracts to the beef concentrate before treated with nitrite did not reduce the mutagenicity. However, the actual mutagenicity of the mixtures of mushroom extracts and beef concentrates treated with nitrite was less than the calculated value. Exceptions were found in button and oyster mushroom extracts on TA98. Thus, it was indicated that some mutagen precursors presented in all concentrates could interact with nitrite and addition of these mushrooms could not reduce the formation of the mutagen precursors.