Immediate hemolysis induced by eight aortic and five mitral valve prostheses was measured in in vitro steady- and pulsatile-flow systems. The mean index of hemolysis (IH) for the aortic valves ranged from 0.010 to 0.031 and 0.012 to 0.035 with steady and pulsatile flow, respectively. The IH for the mitral valves ranged from 0.004 to 0.021 in the steady-flow system, and from 0.011 to 0.021 in the pulsatile system. The Barnard-UCT aortic valve and the Kay-Shiley and Portex-Hammersmith mitral valves produced the least hemolysis in each group. The graphite-benzalkonium-heparin-coated Gott-Daggett valve was the most hemolytic valve in both the mitral and the aortic groups. The degree of hemolysis demonstrated in these experiments appears to be important, particularly since several factors which are likely to have minimized these data have been cited. The calculated daily hemoglobin released from a single prosthetic valve represents a substantial fraction of a human's total hemoglobin clearance capacity and red cell regenerative capability.