Objective: Bites from the brown recluse spider and other arachnids from the genus Loxosceles frequently induce necrotic skin lesions that can be recalcitrant to treatment and disfiguring. The authors used a rabbit model of dermonecrotic arachnidism to address the therapeutic efficacy of intradermal (id) polyclonal anti-Loxosceles Fab fragments (αLoxd Fab) raised against Loxosceles deserta spider venom. Methods: Fab fragments were prepared by papain digestion and affinity chromatography from the IgG fraction of L. deserta antivenom raised in rabbits. Eighteen inbred New Zealand white rabbits were assigned to six groups of three. The rabbits received L. deserta venom (3 μg, id) injections into each flank. Cohorts of rabbits received single id injections (at one venom site/rabbit) of 30 μg αLoxd Fab at different times (T = 0, 1, 2, 4, 8, and 12 hours) after venom injection. In each rabbit the opposite flank was left untreated. As an additional control, one group of rabbits (T = 0) received nonspecific Fab (30 μg, id) in the opposite flank. Dermal lesions were quantified as a function of time through the use of a series of digital photographs and imaging software. In addition, myeloperoxidase (MPO) activity, a measure of neutrophil accumulation, was determined in lesion biopsies. Lesion areas and MPO activities were analyzed by repeated-measures analysis of variance (ANOVA). Results: Lesion areas and MPO activity were markedly reduced when αLoxd Fab was administered very early after venom injections. As the interval between venom inoculation and antivenom treatment increased, the therapeutic benefit of αLoxd Fab decreased. The final time tested that demonstrated therapeutic efficacy of αLoxd Fab was T = 4 hours. Lesion attenuation was no longer apparent when αLoxd Fab was given 8 hours post inoculation. Conclusions: Intradermal administration of αLoxd Fab attenuates Loxosceles-induced dermonecrotic lesion formation when given up to 4 hours after venom inoculation in this rabbit model.