The efficacy of intramuscular (im) administration of sheep Fab and IgG antivenoms was assessed in a mouse experimental model of envenomation by Bothrops asper, in order to test if the more rapid absorption of Fab improves neutralization. Both antivenoms were adjusted to have a similar neutralizing potency in assays involving preincubation of venom and antivenom. Neither antivenom was effective in neutralizing lethality, nor in prolonging the time of death, in mice injected with either 3, 2 or 1.5 LD(50)s of venom by the intraperitoneal (ip) route, in experiments in which antivenoms were administered im immediately after envenomation. Antivenoms were effective in the neutralization of defibrinating activity, even if treatment was performed 30 min after envenomation, with no differences between IgG and Fab. Regarding neutralization of local effects, i.e. myonecrosis and hemorrhage, im administration of antivenoms at a site distant from the venom-injection site was completely ineffective in reducing the extent of local tissue damage. However, partial neutralization of these effects was achieved if antivenoms were administered im at the same site of venom injection, provided treatment was performed immediately after envenomation. Fab antivenom was slightly more effective than IgG antivenom in the neutralization of myotoxicity under these conditions, although a similar efficacy was observed between these antivenoms regarding neutralization of hemorrhagic effect. Our observations do not evidence major differences in the neutralizing ability of Fab and IgG antivenoms when applied by the im route, and do not support the hypothesis that im administration of Fab antivenoms constitutes an effective alternative to treat B. asper envenomations.