To investigate antiherpetic substances from Ganoderma lucidum, various protein bound polysaccharides, GLhw, GLhw-01, GLhw-02, GLhw-03, were isolated by activity-guided isolation from water soluble substances of the carpophores. These substances were examined for their antiviral activities against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) by plaque reduction assay in vitro. Among them, the acidic protein bound polysaccharide, GLhw-02 of a brownish substance, exhibited the most potent antherpetic activity with 50% effective concentrations (EC50) of 300 approximately 520 microg/ml in Vero and HEp-2 cells, and its selectivity indices (SI) were more than 20. GLhw-02 was identified to consist mainly of polysaccharide (approximately 40.6%) and protein (approximately 7.80%) by anthrone test and Lowry-Folin test, and showed the usual molar ratio (C:H:O = 1:2:1) of carbohydrates by elemental analysis. These results suggest that GLhw-02 possesses the possibility of being developed from a new antiherpetic agent.