The physiological effect of porphyran, a sulfated polysaccharides from an edible red alga, was studied in human hepatoma HepG2 cells. Porphyran supplementation significantly decreased apolipoprotein B100 secretion, and the reduction was partly associated with suppression of cellular lipid synthesis in HepG2 cells. This is the first study to elucidate the mechanism of the hypolipidemic effect of porphyran.