Previously, a strong antigenotoxic activity was detected in the extract of an edible green alga, Enteromorpha prolifera (Sujiao-nori in Japanese) (Okai et al 1994 Cancer L ett 87, 25Γ32). The present authors studied the active principle corresponding to the antigenotoxic activity in the extract of this alga. A darkgreen spot was detected on a silica gel TLC of the extract of E prolifera which was further puriΓed by a HPLC system and identiΓed to be a derivative of chlorophyll a, pheophytin a, compared with the retention time of HPLC and absorption spectrum of the standard pheophytin a from Anacystis nidulans. Pheophytin a derived from E prolifera caused a dose-dependent suppressive e β ect on umu C gene expression in Salmonella typhimurium (TA 1535/pSK 1002) induced by 2aminoanthracene and 3-amino-1,4-dimethyl-5H-pyrido[4,3Γ6]indole in the presence of hepatic metabolising enzymes. Furthermore, the standard pheophytin a from A nidulans also exhibited a similar inhibitory e β ect on umu C gene expression system at the same concentrations. These results indicate that pheophytin a is a potent suppressor against genotoxin-induced umu C gene expression in S typhimurium (TA 1535/pSK 1002) probably associated with the carcinogenesis.