Phospholipase A2 is a member of the class of heat-stable, calcium-dependent enzymes catalyzing the hydrolysis of the 2-acyl bond of 3-n-phosphoglycerides. The enzyme has a molecular weight of 30,000 daltons. Phospholipase A2 is activated by Ca2+. It is inhibited by zinc, barium, and manganese ions. Activity values for phospholipase A preparations which are derived from titrimetric assay procedures can be quite dependent on source and type of lecithin, its preparation as a substrate emulsion, other components of the reaction mixture, and the method and instrumentation used.