Following subcutaneous injection of Loxosceles-laeta venom in mice, both the hematocrit and the hemoglobin concentration increased in the peripheral blood. Hemolysis was not detected. Total blood-volume and plasma-volume decreased; fixed globular-volume remained constant. Autopsy showed a generalized edema and ascites, without modifications in total body-weight. These results have shown that mice have a different kind of toxic manifestation compared with those presented in humans. This fact questions the use of mice in assays of protective activity of hyperimmune sera.