Mechanical circulatory support was accomplished in 20 calves (mean, 140 days) and in 5 patients following operation for acquired heart disease (range, 1 hour to 8 days) employing a pneumatically actuated xenograft-valved assist pump interposed between the left ventricular apex and aorta. Following pump implantation in calves, hematocrit and platelets decreased transiently and returned to normal within 14 days. Plasma hemoglobin and erythrocyte mechanical fragility values were elevated for 48 hours. Platelet survival was slightly reduced, but erythrocyte survival values were similar to controls. In patients who received assist pumps, plasma hemoglobin and erythrocyte mechanical fragility were transiently elevated, but rapidly decreased to normal. Thrombocytopenia occurred only in the presence of bleeding and renal failure requiring hemodialysis. Pump flow of the left ventricular assist device was maintained above 2.0 L/min/m2 despite serious arrhythmias. Postmortem examination revealed no evidence of thromboemboli in the clinical patients although anticoagulant agents were not administered.