The protective effect of dietary fiber on human cancer has received great attention during the last decades. Because dietary fiber constitues a large group of complex polysaccharides with various solubilities, degrees of lignification, chemical compositions and structural arrangements, several mechanisms for their effects have been proposed. In this study, in vitro binding capacities of various dietary fibers (potato fiber and glucomannan) and dietary fiber constituents (pectic acid and cellulose) against indirect mutagen 2-amino-3-methyl-3H-imidazo (4,5-f) quinoline (IQ) and direct-acting mutagen sodium azide were investigated. Direct-acting mutagen sodium azide was not adsorbed to the dietary fiber and dietary fiber constituents at 0 degrees C, pH 4.5 and 37 degrees C, pH 7.0. However, indirect-acting mutagen 2-amino-3-methyl-3H-imidazo (4,5-f) quinoline (IQ) were sorbed by them in variable ratios at 0 degrees C, pH 4.5 and 37 degrees C, pH 7.0. The differences between the in vitro binding capacities of the samples at two experimental conditions were found to be statiscially significant (P < 0.01). IQ was not released from the dietary fibers and constituents in distilled water.