An extracellular, nonelastolytic, neutral metalloprotease of Serratia marcescens was purified by sequential ammonium sulfate precipitation, hydroxyapatite adsorption chromatography, flat-bed isoelectric focusing, and Sephadex G-100 gel filtration. The protease preparation had a 280/260 nm absorbance ratio of 1.8, was free of detectable amounts of endotoxin, carbohydrate, phosphorus, and other known extracellular enzymes of S. marcescens, and was homogeneous by Ouchterlony double immunodiffusion and Grabar-Williams immunoelectrophoresis. Crossed immunoelectrophoresis, thin-layer electrofocusing in polyacrylamide gel, and polyacrylamide disc gel electrophoresis showed three to four closely migrating, Coomassie blue-staining components in the protease preparation. However, zymogram analyses of the patterns showed that protease activity was associated with each component and that the protease was, therefore, microheterogeneous. The isoelectric point and sedimentation coefficient of the protease were approximately 5.3 to 5.4 and 4.2S, respectively, and the molecular weight estimated by sodium dodecyl sulfate-polyacrylanide gel electrophoresis and by gel filtration was approximately 52,500 and 44,000, respectively. The pH optimum range, with azocasein as the substrate, was 5.5 to 7.5. The enzyme contained a high percentage genesis of S. marcescens keratitis and pneumonia, therefore, prompted us to develop a purifi-411 on August 1, 2020 by guest