Two Bacillus sp. strains, HR-08 and KR-8102, isolated from soil of the west and north parts of Iran were screened on gelatin agar medium for their ability to produce alkaline protease. The enzymes were active in a wide pH range (6.0-11.0) and stable in the alkaline range (7.0-12.0). The optimum temperatures for the protease from HR-08 and KR-8102 were 65 and 50 degrees C, respectively. The irreversible thermoinactivation of HR-08 and KR-8102 proteases showed that the stability of HR-08 enzyme was higher than that of KR-8102 and the half-lives of these enzymes were 95 and 32 min at 50 degrees C, respectively. In the presence of 10 mM Ca2+, HR-08 retained 100, 90, and 20% of its initial activity after heating for 30 min at 50, 60, and 70 degrees C, respectively. Enzymes were inhibited by phenylmethylsulfonyl fluoride and iodoacetate. After inhibition by iodoacetate, both enzymes were reactivated by dithiothreitol. These data show that the enzymes seem to be thiol-dependent serine alkaline proteases. The enzymes especially from HR-08 were stable in the presence of H2O2, surfactants, and local detergents; their activities were enhanced in the presence of 5 mM Fe2+; and the presence of 5 mM metal ions such as Mg2+, Cu2+, and Mn2+ produced almost no effect.