We previously isolated a unique anticoagulant protein named IX/X-bp (factor IX/factor X-binding protein) from the venom of the habu snake Trimeresurus flavoviridis. We recently determined its primary structure and found that this protein had a structure homologous to the carbohydrate-recognition domains of C-type lectins. Most interestingly, a high homology was found between this protein and botrocetin, an inducer of platelet agglutination found in the venom of the jararaca snake Bothrops jararaca. To examine the possible identity of these proteins, we searched for IX/X-bp-like protein(s) in the venom of B. jararaca. When the venom was subjected to DEAE anion-exchange chromatography, such an activity was eluted separately from that of botrocetin. This activity was purified to homogeneity and designated jararaca IX/X-bp. Jararaca IX/X-bp was a disulfide-linked heterodimer consisting of 16- and 15-kDa subunits, being structurally similar to botrocetin. The respective NH2-terminal amino acid sequences were also very similar. Jararaca IX/X-bp had no botrocetin-like activity. However, this protein did have an activity to bind to factors IX and X and protein S in a Ca(2+)-dependent fashion, that resulted in interference with coagulation, while botrocetin did not. The binding to coagulation factors appeared not to be mediated by the lectin-like activity of jararaca IX/X-bp, because a derivative of factor X free of carbohydrates retained the ability to bind. It is concluded, therefore, that the two proteins isolated from the same venom have different biological activities despite the high degree of structural similarity between them.