There are fluid mechanical factors in all pumping circuits that can conceivably damage erythro-cytes: flaw in tubing is accompanied by a pressure drop and shear stresses; changes in cross section can be accompanied by intense shear stress, turbulence and extremes in pressure; working surfaces (pistons or impellers or rollers) usually induce pressure extremes. There is a considerable body of literature on the hemolysis caused by complete extracorporeal circuits(1) but it is at present not possible to pinpoint the principal sources of hemolysis in systems so complex. Our group in the Mechanical Engineering Department at the University of Minnesota, in cooperation with the Department of Surgery, has undertaken the examination of elementary fluid systems in order to obtain a better understanding of physical factors contributing to hemolysis. The study includes the effect of pressure and pressure fluctuations, the effects of shear stress, the influence of occlusion and of turbulence on hemolysis. The aim is to design experiments in which a single variable appears to be the most likely traumatic agent in the system. This paper describes some of the results obtained in the first year of this continuing program.