Studies are reported on the inhibition of phospholipase A2 (PLA2) from porcine pancreas, cobra (Naja naja) venom, and the P388D1 macrophage-like cell line by human recombinant lipocortin I and bovine lung calpactin I. Membrane vesicles prepared from 1-stearoyl,2-arachidonoyl phosphatidylcholine (PC) and other PCs were utilized as substrate. Binding studies using sucrose flotation gradients showed that both lipocortin I and calpactin I bind to these vesicles although less tightly than to vesicles prepared from anionic phospholipids or fatty acids. Binding to PC was somewhat enhanced by Ca2+. Inhibition of cobra venom PLA2 was not observed when PC vesicles were used as substrate but was when dipalmitoyl phosphatidylethanolamine was used. Both the pancreatic and macrophage enzymes were inhibited when acting on PC. Interestingly, the inhibition of the macrophage enzyme toward PC depended on the fatty acid attached to the sn-2 position of PC with arachidonate greater than oleate greater than palmitate. Inhibition was also highest at low [PC]; these inhibition results can be explained by the "substrate depletion model" (Davidson, F. F., Dennis, E. A., Powell, M., and Glenney, J. (1987) J. Biol. Chem. 262, 1698-1705). Experimental and theoretical considerations suggest that the in vitro inhibition by lipocortins of this macrophage PLA2 from a cell that makes lipocortin and is active in prostaglandin production is due to effects on substrate availability rather than direct inhibition.