The action of an anti-loxosceles serum on in vitro human red blood cell hemolysis and on the development of ulcero-necrotic lesions in rabbit skin, induced by loxosceles laeta venom, was studied. An 81 +/- 3% hemolysis was obtained after 72 h incubation of a 2.5% Rh+ red blood cell solution with the equivalent of one Loxosceles laeta venom gland. This parameter was not modified adding anti-loxosceles serum before, along with or after the venom (79.7 +/- 0.8, 77.3 +/- 2.1 and 80.7 +/- 0.7% respectively). After the intradermic injection of a minimal necrotizing venom dose in rabbits, a skin necrotic lesion appeared. This lesion did not appear if anti-loxosceles serum was injected together with the venom; if the serum was injected one hour after the venom, the resulting skin lesion was inflammatory but not necrotic. It is concluded that anti-loxosceles serum does not inhibit loxosceles venom induced hemolysis and causes a time dependent inhibition on skin necrotic lesions.