Mutagenicities of AF-2, MNNG, 4NQO, aflatoxin B1, benzo [a] pyrene and Trp-P-1, with or without metabolic activation, were inactivated by treatment with human saliva to a great extent in the Ames test with Salmonella typhimurium test strains TA98 and TA100. Mutagenic activities of quercetin, pyrolysates of beef, salmon and sodium glutamate, and condensate of cigarette smoke were also decreased to some extent by saliva treatment, but no significant effect was found on the activity of MMS and pyrolysate of polypeptone. These effects showed individual variations. The inhibition of AF-2 mutagenicity by saliva varied with temperature in TA100 but not in TA98 cultures. Boiled saliva inactivated AF-2 mutagenicity in TA98 to some extent but not in TA100 cultures. Inactivation of AF-2 mutagenicity by saliva treatment was completed within 30 sec. Complex mechanisms may be involved in the inactivation of mutagenicity of carcinogens by saliva, including biochemical reactions with enzymes, vitamins, etc. and/or adsorption with high molecular weight substances in saliva such as proteins, bacterial cells, mucous materials, etc.