The rate of hydrolysis of phosphatidylcholine bilayers by phospholipase A2 may be either enhanced or inhibited by the presence of lysolecithin depending on the experimental conditions examined. To further understand the relationship of lysolecithin to phospholipase A2 activity, the binding of lysolecithin to phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus was examined by fluorescence spectroscopy. The tryptophan emission intensity of the enzyme was enhanced by 70% upon addition of lysolecithin. The binding isotherm for lysolecithin to the phospholipase A2 estimated from the fluorescence change was biphasic, with a clear break in the curve occurring at the critical micelle concentration of the lysolecithin. Several observations suggested that the phospholipase A2 was capable of hydrolyzing the lysolecithin although at a rate far below that of phospholipid hydrolysis. These experiments were repeated using several other species of phospholipase A2, and the results were found to be general among the enzymes except the lys-49 isozyme from A. p. piscivorus which displayed neither the dependence on the critical micelle concentration for binding nor the ability to hydrolyze lysolecithin. These results were used as the basis for a quantitative analysis of enzyme fluorescence changes that occur during the time course of phospholipid hydrolysis and of the mechanism whereby lysolecithin inhibits the hydrolysis of phosphatidylcholine bilayers by phospholipase A2.