An in vitro evaluation of an artificial heart. Interactions between human blood and the Penn State Artificial Heart were examined in vitro to study the effects of various operating conditions on the hematologic response. A dual-loop recirculating flow system that accommodated human blood was developed and blood was subjected for 3 hr to various operating conditions known to alter fluid mechanics in the artificial ventricle. The operating conditions investigated were: 60 beats/min at 50% systolic duration, 60 beats/min at 30% systolic duration, and 90 beats/min at 50% systolic duration. Quantification of plasma free hemoglobin provided a direct indicator of hemolysis in the flow system. Platelet number and beta-thromboglobulin levels were monitored to investigate thrombotic activity, and levels of complement 3a were measured to examine complement system activation. The system was effective in demonstrating the relative hemolytic properties of the operating conditions. Ninety beats/min induced 37% more hemolysis than 60 beats/min at 50% systolic duration, and 50% systolic duration induced 32% more hemolysis than 30% systolic duration at 60 beats/min. There were no statistically significant changes in either platelet number or beta-thromboglobulin levels during the 3 hr recirculation period. Increases were seen in complement 3a levels, but these appeared to be surface-induced and not sensitive to the different operating conditions. These studies demonstrate the usefulness of the flow system in examining the relative hemolytic properties of the artificial ventricle, and suggest that bulk turbulent stresses may play a more important role than laminar wall shear stresses in mediating blood damage in this artificial ventricle. 