Snake venom poisoning is a medical emergency requiring immediate attention and the exercise of considerable judgment. Of the estimated 8,000 bites inflicted by venomous snakes in the United States each year, approximately 6,000 are treated with commercial antivenin. The only commercially available antivenin for North American Crotalidae envenomation is Antivenin (Crotalidae) Polyvalent (equine origin) (ACP; Wyeth Laboratories, Philadelphia, PA). A common complication is the high incidence of hypersensitivity reactions, occurring in more than 75% of patients treated with ACP. To minimize these side effects, a novel, affinity-purified, antigen binding fragment (Fab) antivenom (FabAV) for Crotalidae venom poisoning has been produced from the sera of sheep. The new product is Antivenin Polyvalent Crotalid (Ovine) Fab (Crotab; Therapeutic Antibodies, Inc., Nashville, TN). The current report compares the potencies in mice of FabAV and ACP against venom-induced lethality. The results indicate that FabAV is 3.1-9.6 times more potent than ACP for the prevention of lethality of the nine United States venoms tested. For one of the venoms, Crotalus viridis helleri, FabAV was efficacious while ACP was not.