Fecapentaene-12 (Fec-12), a compound thought to be responsible for much of the mutagenicity in fecal extracts from groups at high risk for colon cancer, was assayed for genotoxic potential in a battery of bacterial and mammalian cell short-term assays. This compound demonstrated significant mutagenic activity with Salmonella typhimurium tester strains TA104, TA100 and TA98, inducing approximately 1400, 700 and 100 revertants/micrograms, respectively. Fec-12 caused dose-dependent increases in unscheduled DNA synthesis in both rat hepatocytes and human fibroblasts, indicating its potential genotoxicity to mammalian as well as bacterial cells. Finally, Fec-12 had the ability to induce neoplastic transformation in mouse BALB/c 3T3 cells in the absence of exogenous metabolic activation.