Members of the spider genus . Loxosceles pose a marked health risk to humans because of the seriousness of the necrotic and systemic effects of their bite, known as loxoscelism. The recent confirmation of . Loxosceles similis in residences of Belo Horizonte in Minas Gerais Province, Brazil increases the local potential risk of loxoscelism at higher levels. The first characterization of the venom from this species showed that its main biological effects had a similar intensity as other species (e.g. . Loxosceles intermedia, . Loxosceles laeta, and . Loxosceles gaucho). Therefore, we wished to further analyse the biological activity of the . L. similis venom as well as the capacity of anti-. L. similis-venom serum to reduce dermonecrotic effects to rabbit skin. Histological analysis of rabbit skin 2, 4 and 8. h after intradermal injection of . L. similis venom demonstrated a dense inflammatory infiltrate, edema, degeneration and necrosis of the skin muscle, dissociation of collagen fibers, and disruption of reticular fibers. Importantly, pre-incubation of the venom with anti-. L. similis-venom serum significantly decreased all of these effects. Anti-. L. similis antivenom generated antibodies that were strongly reactive to . L. similis venom and capable of neutralizing the dermonecrotic effects in rabbits caused by this venom. Moreover, the antivenom significantly reduced the sphingomyelinase activity of . L. similis crude venom. Venoms produced by male and female spiders were equally reactive towards anti-. L. similis and anti-. L. intermedia antivenoms, but female venom induced larger lesions on rabbits. In contrast, female venom acted as an immunization enhancer and protected animals from . L. similis envenomation to a greater degree than male venom. In conclusion, the results shown in this study for . L. similis antivenom merits a more in depth study of its properties, which may become a valuable tool against loxoscelism. © 2012 Elsevier Ltd.