Jararhagin is a 52 kDa hemorrhagic P-III metalloproteinase isolated from the venom of the medically important Brazilian pit-viper Bothrops jararaca. It is a member of the reprolysin family of zinc metalloproteinases containing a catalytic metalloproteinase domain followed by a disintegrin-like and a cysteine-rich domain. The impact of jararhagin on hemostasis has been extensively studied using in vitro and in vivo model systems as well as in clinical studies. Jararhagin-induced hemorrhage is the result of the degradation of sub-endothelial matrix proteins leading to the disruption of the blood vessel endothelium, with accompanying disturbances in platelet function. The versatility of jararhagin is further demonstrated by its direct action on von Willebrand factor, the degradation of fibrinogen, by its inhibition of platelet adhesion to collagen and by its inability to be affected by the plasma inhibitor alpha(2)-macroglobulin. Collagen-induced platelet aggregation is inhibited by jararhagin though the binding of the molecule to the alpha(2) subunit I domain of the platelet surface alpha(2)beta(1) integrin (collagen receptor). Jararhagin also cleaves the beta(1) subunit of the same integrin, inhibiting platelet interaction and ultimately causing impairment of signal transduction. The effect of jararhagin on cell systems other than platelets is evaluated; in fibroblasts, jararhagin functions as a collagen-mimetic substrate and, in endothelial cells, it causes apoptosis and indirectly inhibits cell proliferation by release of angiostatin-like compounds. Jararhagin induces a strong pro-inflammatory response characterized by intense leukocyte accumulation at the site of the injection. Although hemorrhage and edema are a response to the direct effect of jararhagin, jararhagin-induced inflammation and necrosis are dependent on macrophages and key pro-inflammatory cytokines or their receptors. Some data also indicate that the toxin possesses anti-tumorgenic properties. Methods for inhibiting jararhagin are reviewed; this encompasses the use of synthetic peptides to the isolation of naturally occurring mammalian peptides and the development of toxin-specific antibodies through DNA immunisation and monoclonal antibody technologies. The availability of jararhagin makes it an important tool for research into the mechanisms of action of similar toxins, for insights into cellular interactions and for clinical investigations into the treatment of envenomings from B. jararaca.