The effects of heat treated vegetables on mutagenicity were studied using the Salmonella typhimurium system. The mutagens used were 3-amino-1-methyl-5H-pyrido[4,3-b]indole, 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide, 4-nitroquinoline-1-oxide, acridine yellow and 2-aminoanthracene. Most of the heated vegetables unexpectedly showed greater inhibitory activity against the mutagenicity than unheated samples. The activity was increased markedly by heat treatment of water soluble indigestible polysaccharides (IPS). The increase in inhibitory activity due to heat treatment of IPS coincided well with the decrease in their viscosity. Incubating mixtures of mutagens with heated water soluble IPS decreased their affinity for XAD-2 resin. Heating seems to increase the detoxification ability of dietary fibers. © 1992.