Pharmacology. Although two antivenoms are still available for the treatment of crotaline envenomation in some parts of the United States, production of the older product, Crotalinae polyvalent antivenom (equine), or Antivenom Crotalinae Polyvalent (Wyeth-Ayerst), has ceased, and it has been replaced by the newer Crotalinae polyvalent immune Fab (ovine), or CroFab (Protherics). To produce the polyvalent Fab antivenom, sheep are hyperimmunized with pooled venom from four North American snakes: Crotalus adamanteus, Crotalus atrox, Crotalus scutulatus, and Agkistrodon piscivorus. Papain then is added to the pooled serum product collected from the donor animals to cleave the immunogenic Fc fragment from the IgG antibody. The result is an affinity-purified Fab fragment antivenom. After administration, the antivenom is distributed widely throughout the body, where it binds to venom. Some remaining stocks of the Wyeth product may be found in parts of the United States.