Anti-tumorigenic action of compounds in some seaweed was evaluated by using selective cytotoxicity against a transformed mouse 3T3 cell (SV-T2) and its normal counter part. The extracts of seaweeds (Wakame, Konbu, Mirin, Aosa, and Sujiaonori) were added to the cells in 96 well microplate. The viability of the cell was measured at 24 hr after sample addition. Of the five weeds, the ethanol extracts from Sujiaonori exhibited the selective cytotoxicity. The ethanol extracts from Sujiaonori was applied to gel permeation chromatography on Sephadex G-50. The anti-cancer activity was detected in two fractions with high molecular weight near void volume and with small molecular weight. The high molecular weight fractions had absorbance at 280 nm, suggesting that they contained proteineous substances. The small molecular weight fractions were estimated to be around 500 Dalton.