In order to isolate substances that inhibit the hemolytic activity of human serum against erythrocytes, we have evaluated whole plants of the Orostachys japonicus species with regard to its anti-complement activity, and have identified its active principles following activity-guided isolation. A methanol extract of the O. japonicus, as well as its n-hexane soluble fraction, exhibited significant anti-complement activity on the complement system, which was expressed as total hemolytic activity. A bioassay-guided chromatographic separation of the constituents resulted in the isolation of three known compounds 1-3 from the active n-hexane fraction. The structure of these compounds were analyzed, and they were identified as hydroxyhopanone (1), beta-sitosteryl-3-O-beta-D-glucopyranosyl-6'-O-palmitate (2), and beta-sitosteryl-3-O-beta-D-glucopyranoside (3), respectively. Of these compounds, compound 2 exhibited potent anti-complement activity (IC50= 1.0 +/- 0.1 microM) on the classical pathway of the complement, as compared to tiliroside (IC50= 76.5 +/- 1.1 microM), which was used as a positive control. However, compounds 1 and 3 exhibited no activity in this system.