Phospholipase A₂ (PLA₂) activity has now been identified in rheumatoid synovial fluids. This PLA₂ is a calcium-requiring protein of MW 11,000 with a neutral pH optimum. Its ativity was inhibited by high concentrations of Mg²⁺, and by the active site-directed histidine reagent p-bromophenacyl bromide. Ionic and non-ionic detergents, or the sulfhydryl reagent dithiothreitol caused loss of enzyme activity. Synovial fluid PLA₂ did not interact with sulphated mucopolysaccharides such as heparin or chondroitin sulphate. Release and sequestration of PLA₂ in the joint space may contribute to the characteristic rheumatoid inflammatory changes.