Abstract The antigenotoxic activities of chitin and chitosan were studied using sister chromatid exchange assay by examining the adsorption of four kinds of mutagens. These two dietary animal fibers showed similar patterns in reducing the genotoxicity of aqueous solutions of the hydrophobic mutagens, 4-nitroquinoline- N-oxide and dinitropyrene, in distilled water. Under similar conditions, the antigenotoxic activities of chitin and chitosan for mitomycin C were 87 and 0%, and those for adriamycin were 47 and 78%, respectively. In addition, the antigenotoxic activity of both fibers for MMC was affected by the pH value of the aqueous solution between 2.5 and 7.2, but that of ADM was not. The results demonstrate that chitin and chitosan may have protective effects against environmental mutagens by adsorbing them in ionic and nonionic solutions.