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The concentration of free Ca²⁺and the composition of nonsubstrate phospholipids profoundly affect the activity of phospholipase C δ1 (PLCδ1). The rate of PLCδ1 hydrolysis of phosphatidylinositol 4,5-bisphosphate was stimulated 20-fold by phosphatidylserine (PS), 4-fold by phosphatidic acid (PA), and not at all by phosphatidylethanolamine or phosphatidylcholine (PC). PS reduced the Ca²⁺ concentration required for half-maximal activation of PLCδ1 from 5.4 to 0.5 μm. In the presence of Ca²⁺, PLCδ1 specifically bound to PS/PC but not to PA/PC vesicles in a dose-dependent and saturable manner. Ca²⁺ also bound to PLCδ1 and required the presence of PS/PC vesicles but not PA/PC vesicles. The free Ca²⁺concentration required for half-maximal Ca²⁺ binding was estimated to be 8 μm. Surface dilution kinetic analysis revealed that the K_m was reduced 20-fold by the presence of 25 mol % PS, whereas V_max …