The effects of cabbage juice on the mutagenicity of heterocyclic amines (Trp-P-1, Trp-P-2 and IQ)were investigated. The mutagenicity was measured by the Ames method, using S. typhimurium TA 98in the presence of S-9 mix. Cabbage juice was separated into a high-molecular-weight fraction (F-I)and two low-molecular-weight fractions (F-II and F-III) by gel filtration with Sephadex G-25. The FI fraction had higher desmutagenicity than the F-II fraction toward Trp-P-1, Trp-P-2 and IQ. However, the F-III fraction had no desmutagenicity toward these heterocyclic amines. In the present study, high peroxidase activity was detected in the F-I fraction. Since commercial horseradish peroxidase shows high desmutagenicity toward heterocyclic amines, the desmutagenic factor in the F-I fraction is presumed to have been peroxidase. Glucose, fructose and sucrose were observed in FII fraction. These saccharides showed moderate desmutagenicity toward Trp-P-1 and Trp-P-2, but showed low desmutagenicity toward IQ.