Protein kinase C displays high apparent cooperativity in its activation by phosphatidylserine. This contribution uses a novel approach to address the physical basis for this apparent cooperativity. We examine the binding of protein kinase C betaII to large unilamellar vesicles as a function of increasing mole fraction phosphatidylserine and as a function of increasing total lipid concentrations. Binding data are subjected to an analysis, described in the Appendix, that allows calculation of the fractional saturation of phosphatidylserine binding sites with this ligand. This analysis reveals that (1) protein kinase C betaII binds approximately eight phosphatidylserine molecules and (2) the binding of each lipid is not cooperative. Rather, the apparent cooperativity observed in protein kinase C's interaction with multiple phosphatidylserine molecules arises from effects specific to the interaction of a multivalent macromolecule with multiple membrane-associated ligands. Nor does diacylglycerol, which has been previously shown to dramatically increase protein kinase C's affinity for phosphatidylserine-containing membranes, induce cooperativity. Thus, protein kinase C binds multiple phosphatidylserine molecules in the absence of interaction between potential binding sites. The method presented for determining the stoichiometry and cooperativity in the interaction of protein kinase C with phosphatidylserine is applicable to any multivalent molecule binding to monovalent ligands incorporated into lipid membranes.