A thermostable extracellular protease named protease RH-1 was purified from a thermophilic Geobacillus sp. YMTC 1049 isolated from a hotspring in Rehai, Tengchong, Yunnan Province, China. Protease RH-1 was a serine protease since it was inhibited by 10 mm PMSF. It was a monomeric enzyme with molecular weight of 59.2 kDa. The protease showed the highest activity at 85 degrees C at pH 7.5 and was stable for 10.0 h at 65 degrees C. The activity of protease RH-1 was enhanced by 10 mM EDTA and EGTA. In addition, the activity of the protease can be activated by Ca2+ and Mg2+ but inhibited partially by Ba2+, Zn2+, Pb2+, Co2+, Mn2+ and Cu2+. It was noteworthy that protease RH-1 was resistant to denaturation by SDS, dithiothreitol, urea and guanidine hydrochloride (GdnHCl). Therefore, protease RH-1 was greatly different from those isolated from other strains of Geobacillus and may be applied in industrial processes that are performed under high temperature, in alkaline medium, or with the presence of denaturants. (C) 2006 Elsevier Inc. All rights reserved.