Previous studies in our laboratory demonstrated the repressive effect of seaweeds (Laminaria sp., Sargassum fulvellum, Eisenia bicyclis and Gelidium sp.) against D-galactosamine (D-GalN)-induced hepatopathy. However, the mechanism by which these four seaweeds attenuate the D-GalN-hepatopathy has not been completely clarified. This study was carried out to determine the constituents of these seaweeds that protect rats against the D-GalN-hepatopathy. Male Wistar rats were fed for 8 d diets containing 5% seaweeds with or without the antibiotic neomycin (NEO) in experiment 1, and typical seaweed dietary fibers (laminaran, fucoidan, alginate, agar and κ-carrageenan) of these seaweeds in experiment 2. On the 7th day, the rats were treated with D-GalN (1,900 mg in experiment 1 and 800 mg/kg in experiment 2) and then sacrificed 24 h after the injection of D-GalN. Their serum transaminase (aspartate and alanine aminotransferases: AST and ALT) activities were then determined. In experiment 1, the serum AST and ALT levels in the rats fed the four kinds of seaweeds without NEO were significantly low in comparison to that of the control group, but those with NEO were not significantly different among the groups. In experiment 2, the serum AST and ALT levels in the rats fed fucoidan were significantly low in comparison to those of the other groups fed the dietary fibers and the control. These results suggest that the protective effect of the three kinds of brown seaweeds Laminaria sp., Sargassum fulvellum and Eisenia bicyclis against D-GalN-hepatopathy was caused at least in part by fucoidan.