Experimental purpura was produced in the hamster by administration of anti-platelet serum obtained from rabbits previously injected with hamster platelets. Spontaneous petechiae and generalized bleeding were observed. The derangement in the hemostatic mechanism has been analyzed by study of the changes in blood, bone marrow and vessel walls. The platelet count in peripheral blood fell from 9.02 ± 0.85 (x 105) to 0.66 ± 0.32 (x 105) at 24 hours after 2.0 ml. intravenous injection of antiplatelet serum. The red cell and hemoglobin values dropped to 50 per cent before death related to generalized bleeding occurred. Significant changes were seen in the megakaryocytes of the bone marrow. The bleeding time and clot retraction were extended. Evidence for a defect in the vessel wall has been shown by the microelectrode, moccasin snake venom and negative pressure tests. The cause of bleeding has been postulated as a double defect resulting from a decrease of platelets in the circulation and an alteration in the integrity of the vessel wall or perivascular supporting sheath.