Optimum reaction conditions by pH-stat studies of lipase fromcandida cylindraceatowards triolein emulsions were determined. These conditions are no less than 40 mMcalcium ion and no more than 0.3Msodium chloride in 10 mM (tris-hydroxymethyl) amino methane (THAM) buffer at pH 7.45. Comparison of the enzyme activity and relative amount of product formed as a function of calcium ion concentration imply a thermodynamic as well as a kinetic role for calcium. Increasing the THAM concentration to 0.5Mappears to facilitate the deactivation of the enzyme by the free fatty acid produced in the reaction.