An amateur herpetologist developed chills, diaphoresis and generalized urticaria 5 h after receiving Antivenin (Crotalidae) Polyvalent (Wyeth) for treatment of rattlesnake (Crotalus atrox) envenomation. The patient had been bitten 8 mo earlier by a copperhead (Agkistrodon contortrix mokeson) resulting in minimal envenomation. He also had been skin testing himself for 6 mo with both diluted Crotalus atrox venom and Antivenin (Crotalidae) Polyvalent (Wyeth) to determine how sensitive he was to antivenom and how resistant he was to the effects of venom. Unusual reactions to antivenom during the treatment of snake-bite victims, such as amateur herpetologists or snake fanciers, with access to both venom and antivenom should raise questions about unusual means of sensitization.