Intravascular haemolysis was studied in a randomized series of 35 patients examined 12--24 months after combined mitral and aortic valve replacements with either the Björk-Shiley (pyrolytic carbon) tilting disc valve (19 patients) or the Lillehei-Kaster pivoting disc valve (16 patients). Red blood cell count, haemoglobin concentrations, and serum bilirubin varied within normal ranges with a few exceptions. Haptoglobin was absent or reduced in 86%, while elevated values for serum lactate dehydrogenase were found in 82%. The increase in serum lactate dehydrogenase activity was moderate and indicated a normal or only slightly reduced red cell lifespan in most patients. Although the intravascular haemolysis was of little clinical significance, the increase in lactate dehydrogenase was significantly higher in patients with the Lillehei-Kaster prosthesis than in patients with the Björk-Shiley prosthesis, indicating a slightly shorter lifespan in the former group. For both models, however, intravascular haemolysis was less severe than previously reported after combined aortic and mitral valve replacement with ball valve prosthesis.