Macrophage elastase was purified from conditioned media from alveolar and thioglycollate-elicited peritoneal macrophages. The enzyme was purified to apparent electrophoretic homogeneity by preparative isoelectric focusing after a purification step consisting of low ionic strength dialysis and sequential batch fractionation on DEAE-Sephadex A-50. The proteinase activities isolated from alveolar and peritoneal macrophages showed the same physical and biochemical properties. This fact suggests that the same enzyme activity is present in rat macrophages of two different anatomical sites. The molecular weight and isoelectric point of the enzyme were estimated to be 22,500 and 8.3, respectively. The enzyme, characterized as a metallo proteinase, had elastolytic activity, as well as activity toward Suc-(Ala)3-NA. It is inhibited by o-phenanthroline, chicken ovoinhibitor, and EDTA, but not by phenylmethylsulfonyl fluoride or soybean trypsin inhibitor. The macrophage enzyme possesses biochemical and biophysical properties different from the rat pancreatic and granulocyte elastases (which are serine proteinases), and from the metallo proteinase with elastolytic activity isolated from rat platelets.