The purpose of this investigation was to determine the inhibition on $Na^+,;K^+$-ATPase, cyclic AMP phosphodiesterase and platelet activation by marine sterols isolated from octocorals. Three marine polyhydroxysterols, 7${alpha},;8{alpha}-epoxy-3b{eta},;5{alpha},;6{alpha}-trihydroxycholestane (1),;24-methyl-7{alpha},;8{alpha}-epoxy-3{eta},;5{alpha},;6{alpha}-trihydroxycholest-22-ene (2),;and;7{alpha},;8{alpha}-epoxy-3{eta},;5{alpha},;6{alpha}-trihydroxycholest-22-ene (3)$, were isolated from the Gorgonian Acabaria undulata. Five marine sterols(compound 4, 5, 6, 7, 8) were isolated from the soft coral Alcyonium gracillimum. Three marine polyhydroxysterols (1, 2, 3) and pregna-1. 20-diene-3-one (8) exhibit a potent inhibitory effect on rabbit platelet aggregation induced by collagen and thrombin. Those polyhydroxysterols also exhibit a potent inhibitory effect on cyclic AMP phosphodiesterase. Compound 6 with an unusual cyclic enolether exhibit a inhibitory effect on $Na^+,;K^+$-ATPase.