A new centrifugal blood pump system has been developed for left ventricular bypass by the addition of non-thrombogenic blood surface materials and an ultrathin-walled cannula for the retrograde cannulation of the left ventricle. Partial LV bypass at 3 to 6 L/min was undertaken in 55 calves without thoracotomy. In 20 it was continued for 24 hours, with 13 survivors who were eventually sacrificed. Eleven of the last 14 experiments were completed without mishap. Heparin was employed only during pump insertion. Hematologic changes were limited to moderate platelet depression, and tolerable hemolysis (average serum level 21 mg% in the last 13 experiments). Normal clotting parameters and the absence of significant fibrin split product formation correlated with the absence of gross thrombosis and few minor renal emboli observed at autopsy. This pump system appears to have several advantages over previously described equipment for LV bypass.