To study the effects of loxosceles laeta venom on red blood cells and the possible factors involved in hemolysis during arachnidism, in vitro models were designed to measure the role of loxosceles venom, calcium, complement and antibodies in the mechanism of red blood cell destruction. The degree of basal hemolysis was measured in a 5% suspension of group O, Rh (+) red blood cells in pH 7.4 buffer. In a similar suspension spider venom was added in amounts equivalent to one venom gland. After 72 hours of incubation, basal hemolysis was 5.59 +/- 2.04% which increased to 26.01 +/- 7.9% adding venom (p < 0.001). Adding calcium to the incubation medium, hemolysis increased to 88.5 +/- 7.16% (p < 0.001). Incubating red blood cells with control human serum and venom, hemolysis was 14.58 +/- 2.42%, which decreased significantly to 6.85 +/- 3.35% when serum was heat inactivated; this demonstrates an effect of the presence of complement. We did not find antivenom antibody production in patients with arachnidism 10, 15 or 30 days after the spider bite. It is thus demonstrated that loxosceles laeta venom has a direct lytic action on red blood cells that is calcium and complement dependent and is not mediated by antibodies or other substance.