Hydrolysis of Escherichia coli membrane phospholipids by pancreatic phospholipase A 2 was inhibited by lipocortin from human monocytes in a substrate dependent manner. Inhibition was completely overcome at substrate concentrations above 250 pM. Lipocortin also inhibited partially purified preparations of two intracellular phospholipases A 2 isolated from rat liver mitochondria and rat platelets when these enzymes were assayed at low micromolar concentrations of phosphatidylethanolamine. Inhibition gradually decreased with increasing substrate concentrations both for pancreatic and platelet phospholipase A 2 and became completely abolished above 15 and 50/tM phosphatidylethanolamine, respectively.