The mutagenicity of bile acids was detected by a fluctuation test using Salmonella typhimurium TA100 and TA98 as tester strains. Cholic acid, chenodeoxycholic acid, deoxycholic acid and ursodeoxycholic acid were mutagenic in this test while lithocholic acid was not. The mutagenicity of the bile acids on a molar basis was roughly one-fourth that of methyl methanesulfonate, a moderately potent mutagen. Epidemiological studies have shown a high correlation between levels of bile acids excreted and colon cancer. However, no evidence has previously been reported showing that bile acids are mutagenic. Our results suggest that bile acids may be important in the etiology of colon cancer.