Some structural and catalytic properties of the extracellular serine protease thermitase, purified by chromatography on porous glass, are reported. The crystal data and the high elastinolytic action point to possible relationship of thermitase with pancreatic elastase. Subsite-mapping studies clearly indicate, however, that thermitase contains an extended binding region and is closely related to the subtilisin group. The enzyme shows maximal stability between pH 6.0 and 7.5 and maximal activity between pH 7.5 and 9.5. The larger the substrate, the higher is its temperature optimum (60 degrees C for esterolysis, 85 degrees C for proteinolysis). The stability of thermitase is significantly improved by acetates and chlorides at 1 M concentration. Besides its high hydrolytic action on soluble proteins thermitase is capable for efficient degradation of the insoluble proteins elastin and collagen.