Approximately 700 people die in Mexico each year from scorpion stings. The only useful therapy available is antiserum obtained from horses immunized with macerates of venomous gland from scorpions of the genus Centruroides. We report the results of experiments conducted with mice and rats in order to evaluate the relevant components of the venom from Centruroides noxius in the induction of a protective response against scorpion envenomation, either in vivo or in vitro. Gland macerates of whole telsons (stinger), soluble venom extracted by electrical stimulation, toxic fractions from gel filtration on Sephadex G-50 and highly purified toxin 2 from this scorpion venom were all used to produce hyperimmune mice and rats, which were challenged in vivo with the equivalent of the lethal dose 50% (LD50) of soluble venom, or their sera were prepared for in vitro neutralization experiments using non-immunized animals. The maximum neutralizing capacity (100%) was obtained when soluble venom was used as antigen, while purified toxin 2 produces 80% survival in vivo. The neutralizing capacity of murine antisera evaluated in vitro was: sera antifraction II > antitoxin 2 > antitotal venom > anti-gland macerates of whole telsons. Two additional aspects were further investigated in the present work. One is the demonstration by immunoblotting that proteins corresponding to the electrophoretic mobility of toxins known to block sodium channels are highly immunodominant in this venom. Second, there is a strong cross-reactivity of antisera produced with Centruroides noxius when assayed against venoms from other dangerous species of Centruroides scorpions from Mexico, but not against the Israeli scorpion Leiurus quinquestriatus. Finally, the immunodominance of toxic fractions in the immune response was observed either with immunization using Freund's adjuvant or by means of adsorption to nitrocellulose membranes. This latter vehicle was shown to be an excellent detoxifying agent, without changing the immunogenicity of the toxins, as might occur with chemical treatment of these neurotoxic peptides.