An extreme thermophile that produces a heat-stable proteinase was isolated from hot-spring water and classified as Thermomicrobium sp. KN-22 (growth temperature, 50-83 degrees C; and optimum growth temperature, 70 degrees C). The proteinase was purified from the culture broth of this strain by fractionation with ammonium sulfate, chromatography on columns of DEAE-cellulose and CM-Sepharose CL-6B, and HPLC on TSKgel CM-5PW. The purified enzyme gave a single band on SDS-polyacrylamide gel electrophoresis and a single peak after HPLC (yield 8.8%). The enzyme had maximum activity at pH 8.5 and at 75 degrees C and it was stable up to 60 degrees C. The molecular weight of the enzyme was 35,000 by SDS-PAGE. Since the enzymatic activity was completely inhibited by EDTA, o-phenanthroline, and phosphoramidon, it appears that the enzyme is a metal proteinase.