1. The inhibitory powers of thirty two samples of steroids and their related compounds on the theta-toxin induced hemolysis were assayed. Apparent I50 value of cholesterol, the most potent inhibitor amoung them, was 0.022 muM. 2. Inhibitory powers of sterol acetates were not more than one tenth of those of the corresponding sterol, and steroids without 3-hydroxyl group hardly inhibited the hemolysis. These results suggest that 3-hydroxyl group of sterols plays an important role in the inhibition. 3. The inhibitory power of sterols were strongly affected by steric situation of 3-hydroxyl group for the face of their tetracyclic rings. From these results it is suggested that theta-toxin binds to beta-side of the tetracyclic rings but not to alpha-side. 4. A methylsterol and pentacyclic triterpens with 3-hydroxyl group also inhibited the hemolysis, but steroids and their related compounds either with a hydrophylic or without a hydrophobic group at their ring D had little, if any, inhibitory power.