Recently, the complete amino acid sequences have been determined for several snake venom metalloproteinases from the genera Crotalus, Trimeresurus and Lachesis of the Crotalidae family. Among these are both hemorrhagic and nonhemorrhagic metalloproteinases. Despite differences in their molecular weights and activities, they all appear to be related through a single ancestral gene, as observed by the comparison of their amino acid sequences. None of these proteins bear significant similarity to any other known protein, with the exception of the zinc binding site demonstrated in thermolysin and other metalloproteinases. Thus we propose that these proteins define a new family of zinc metalloproteinases which is likely to consist of (but is not necessarily limited to) many of the zinc metalloproteinases found in snake venoms.