PhospholipaseA 2 (PLA 2 )-mediated hydrolysis of membrane phospholipids was measured by ellipsometry, and the inhibition of this process by annexinV was studied. Planar membranes, consisting of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine (PC/PE/PS; 54:33:13, on molar basis), were degraded by pancreatic PLA 2 , and the rate of hydrolysis was limited to about 0.7%/min. The influence of graded coverage of the membrane with annexinV was studied. The degree of PLA 2 inhibition was nonlinearly related to the amount of membrane-bound annexinV, and binding of only 12% and 54% of full membrane coverage resulted in, respectively, 50% and 93% inhibition. These findings indicate that the inhibition of PLA 2 -mediated hydrolysis by annexinV cannot be simply explained by shielding of phospholipid substrates from the enzyme. Moreover, the present results leave room for a role of endogenous annexinV in regulating phospholipid turnover in the plasma membrane of parenchymal cells such as cardiomyocytes.