The anti-herpes simplex virus type 1 (HSV-1) activity of 15 oleanan-type triterpenoides including glycyrrhizin and its sapogenol was examined and their structure-activity relationships were discussed. Although glycyrrhizin which exhibited in vivo efficacy against HSV-1 replication showed moderate in vitro anti-HSV-1 activity, its sapogenol, glycyrrhetic acid, showed 10 times greater action than glycyrrhizin. Therefore, the in vivo anti-HSV-1 activity of glycyrrhizin administered orally could be reasonably attributed to glycyrrhetic acid generated by hydrolysis by intestinal bacteria. Since the activity of soyasapogenol A was less than 1/20 of that of soyasapogenol B, the hydroxylation at C-21 seemed to reduce anti-HSV-1 activity. Since kudzusapogenol A, abrisapogenols B and C lacked the activity, the C-29 hydroxy group would eliminate anti-HSV-1 activity. On the other hand, since the methylesters of kudzusapogenol B and glycyrrhetic acid exhibited greater action, a methoxy carboxy group at C-20 might enhance activity.