1. It has been shown that factor V accelerates the rate of conversion of prothrombin into thrombin by activated factor X, and that this activity does not depend on the presence of phospholipid. 2. Although the mechanism by which factor V increases the rate of reaction is not clear, it seems unlikely that it is converted into an enzyme. 3. The rate of conversion of prothrombin by purified activated factor X and factor V was increased threefold in the presence of phospholipid, and it is suggested that the lipid acts by adsorbing the proteins on its surface and hence increasing the local concentration of the reactants.