The binding of cholesterol by pneumolysin, alveolysin, and streptolysin O has been demonstrated. The properties of the cytolysin-cholesterol interaction parallel those of cytolysin-erythrocyte interaction in that the reaction is rapid, temperature independent, decreased at elevated pH, and shows the same specificity with respect to other related sterols. However, oxidized or p-hydroxymercuribenzoate-treated thoxin showed no decrease in cholesterol-binding activity, whereas the ability of cytolysin to bind to erythrocytes was modified by such treatment.