Metalloproteinase from the venom of Bothrops asper (proteinase G) is a glycoprotein with 1% neutral hexose and 3.5 moles of sialic acid per mole of protein. It hydrolyses a number of protein substrates such as casein, hemoglobin, gelatin and fibrinogen, whose alpha chain is degraded preferentially. The pH optimum of hydrolysis of casein is approximately 9.5. The protease is devoid of hemorraghic, esterolytic and amidolytic activities. The proteolytic activity of the enzyme increases by about 20% in the presence of 0.2 mM Ca2+ and Mg2+. Among the other ions tested, only Cd2+ and Fe2+ markedly decreased its activity. EDTA and cysteine are also strong inhibitors. In the presence of Ca2+ and EDTA, Zn2+ ions restored 50% of the activity. The amino acid composition shows fewer acidic residues than in related proteinases from other snake venoms.