An antihyperlipidemic effect was observed by intraperitoneal injection of the seaweed Monostroma nitidum extract. Hyperlipidemia was induced by intraperitoneal injection of Triton WR-1339 $(60;{mu}g/g-body;weight)$ into a mouse. Maximum level of blood cholesterol was reached at 20 hours after Triton injection. By simultaneous injection of the M. nitidum extract $(30;{mu}g/g-body;weight)$ with the Triton into each right and left sides of the peritoneal cavity, levels of total cholesterol and low density lipoprotein were decreased to $24\%;and;14\%$ compared to Triton injection only. For histochemical changes, hepatic tissues obtained at 40 hours after injection of the Triton and the M. nitidum extract were fixed in fromol-calcium solution. The meshlike cytoplasms disappeared and hapatic plates were recovered in mice injected with the M. nitidum extract. Numbers of lipid drops and cholesterol particles also decreased in the portal space of the hepatic cytoplasm. This indicates that the accumulation of lipid, including cholesterol, caused by Triton was prevented by the antihyperlipidemic effect of extract from the seaweed M. nitidum.