The cell wall sulfated polysaccharide of the red microalga Porphyridium sp. exhibited impressive antiviral activity against herpes simplex virus types 1 and 2 (HSV-1 and -2) both in vitro (cell culture) and in vivo (rats and rabbits). Depending on the concentration, this polysaccharide completely inhibited or slowed down the development of the cytopathic effect in HSV-infected cells, but did not show any cytotoxic effects on vero cells even when a concentration as high as 250 microg/ml was used. There was indirect evidence for a strong interaction between the polysaccharide and HSV and a weak interaction with the cell surface. When tested in vivo, Porphyridium sp. polysaccharide conferred significant and efficient protection against HSV-1 infection: at a concentration as low as 100 microg/ml, it prevented the appearance and development of symptoms of HSV-1 infection in rats and rabbits. The polysaccharide did not exhibit any cytotoxic effects at a concentration of 2 mg/ml in vivo.