Several snake venoms contain procoagulant proteins that can activate prothrombin. We have purified pseutarin C, a prothrombin activator from the venom of the Australian brown snake (Pseudonaja textilis). It converts prothrombin to thrombin by cleaving both the peptide bonds Arg(274)-Thr(275) and Arg(323)-Ile(324), similar to mammalian factor Xa. It is a protein complex (approximately 250 Kd) consisting of an enzymatic and a non- enzymatic subunit. These subunits were separated by reverse phase HPLC and their interactions with bovine factor Xa and factor Va were studied. The enzymatic subunit of pseutarin C has an approximately 13 fold higher affinity for bovine factor Va (K(d) of 11.4 nM for pseutarin C enzymatic subunit--bovine factor Va interaction as compared to a K(d) of 147.4 nM for the bovine factor Xa-Va interaction). The non-enzymatic component, however, was unable to activate bovine factor Xa. N-terminal sequence analysis of the catalytic subunit of pseutarin C showed approximately 60% homology to mammalian factor Xa and approximately 78% homology to trocarin, a group D prothrombin activator from Tropidechis carinatus venom. Structural information for the non-enzymatic subunit of pseutarin C was obtained by amino terminal sequencing of several internal peptides. The sequence data obtained indicates that the non-enzymatic subunit of pseutarin C has similar domain architecture like the mammalian factor Va and the overall homology is approximately 55%. Thus pseutarin C is the first venom procoagulant protein that is structurally and functionally similar to mammalian factor Xa-Va complex.