Two kinds of mechanical valve, St. Jude Medical (SJM) and Björk-Shiley (B-S), in patients with single valve replacement have been evaluated on a view point of intravascular hemolysis. Cases were consisted of 78 SJM and 32 B-S for the mitral and 12 SJM and 36 B-S for the aortic positions. Serum LDH, indirect bilirubin, GOT, hemoglobin levels and reticulocyte count were pursued in a given schedule, especially on the 21st postoperative day. A follow-up study on cases with abnormally high level of LDH (over 800 W.U.) at the time of hospital discharge was also performed. Paravalvular leakage detectable on the echocardiographical study was denied in all cases. The case with SJM valve replacement on the mitral position revealed a higher level of LDH, GOT and a large count of reticulocyte than those in the B-S valve. Insignificantly was found, however, in the same study with replaced valve on the aortic position. There was no correlation between the severity of hemolysis and the size of prostheses. The level of LDH, remained in high on discharge declined gradually thereafter during postoperative observation. As a whole, the result of this study supports the conventional valve selection and usage in our clinic. The patient with subclinical hemolysis after valve replacement should be placed on a schedule of close observation.