The paper stresses the similarity between pathological changes produced in dogs by estradiol benzoate and anti-platelet serum. Estradiol benzoate when administered in large doses (0.1 mg per kilo) produces primarily a thrombocytopenia after a short period (4 to 6 days). As a consequence of the lack of platelets in circulation (1/3 to 1/10 from the normal volume), a picture of purpura appears with predominance of intestinal hemorrhages followed by severe anemia (8 to 15 days). When active anti-platelet serum is administered a thrombocytopenia is rapidly obtained (12 to 48 hours). Intestinal purpuric lesions are very conspicuous and hemorrhages with acute anemia are always present. These results show the identity of the mechanism of anemia produced by estradiol benzoate and anti-platelet serum. The pratical value of determinations of occult blood in feces of patients with thrombocytopenic purpura is emphasized, as an aid to diagnosis and observation od crises of the Werlhof's disease.