Phospholipase A2 from cobra venom (Naja naja naja) is a homogeneous, heat-stable enzyme that has a monomer molecular weight of only 11,000 and contains one histidine and one tryptophan residue. This enzyme acts optimally on phospholipids contained in mixed micelles with the nonionic detergent Triton X-100; the interactions of this detergent with phospholipid in the mixed micelles have been elucidated with 1H- and 13C-NMR. In reacting, the enzyme first associates with the mixed micelle and then exhibits "surface dilution kinetics" in its reaction with substrate. Recent studies show that various reagents completely inactivate this phospholipase A2 resulting in the modification of the histidine in only one-half of the enzyme molecules. These results suggest that the histidine residue, which is essential for activity, exhibits "half-site reactivity." These and other experiments are interpreted in terms of a model that suggests that the monomeric enzyme forms an asymmetric dimer or higher order aggregate at the lipid-water interface. The studies which are described on the interaction of the phospholipase A2 with mixed micelles serve as a general model system for understanding detergent effects on the assay of lipid enzymes.