From the butanol fraction of the starfish Asterina pectinifera MULER et TROSCHEL (Asteriidae), we have isolated a new component, 5α-cholest-7-en-3β-ol. Its antigenotoxic and antimutagenic activities were examined by the SOS chromotest with Escherichia coli PQ37 and by Ames test with Salmonella typhimurium TA1538, respectively. 5α-Cholest-7-en-3β-ol showed potent antigenotoxic activity against the mutagens, both MNNG and NQO. For 100% of antigenotoxicity, the concentration of the compound applied against MNNG and NQO were 10 μg and 5μg per reaction tube, respectively. Its antimutagenic activity with S. typhimurium TA1538 against the mutagen MNNG was very effective. When its concentrations were varied from 1 up to 10 μg dose per plate, the inhibition ratio of revertant CFU of TA1538 per plate was increased accordingly, from 25.2 to 99.2%. These results suggest that 5α-cholest-7-en-3β-ol possesses antigenotoxic and antimutagenic activity and might be useful as a chemopreventive agent.