The inhibition by filipin in the extrinsic blood clotting system takes place at two sites: (1) at the interaction of factor VII, tissue thromboplastin and Ca++, and (2) at the prothrombinase level. Both formation and enzymatic activity of the prothrombinase are inhibited.The cholesterol content of the lipid that is used to form the prothrombinase complex determines the degree of inhibition by filipin. The inhibition increases the higher the sterol content. The adsorption of factors Xa and V onto lipid surface is enhanced by filipin. This increased adsorption does not lead to an increase in prothrombinase production.