A study was undertaken to evaluate hemolysis and subsequent renal damage in 14 patients undergoing cardiopulmonary bypass (CPB) surgery. In all patients, free haptoglobin disappeared completely 30 to 90 minutes into CPB, while free hemoglobin (Hb) levels increased progressively. The NAG index and alpha 1M index also increased progressively, indicating renal tubular injury due to hemolysis (Study 1). An additional 20 patients were monitored intraoperatively for plasma free Hb levels by a newly developed colorimetric method using a haptoglobin coated strip. Free Hb levels during CPB exceeded 30 mg/dl in 14 patients, who were immediately given haptoglobin. This treatment eliminated plasma free Hb within 30 minutes, and effectively prevented hemoglobinuria. Haptoglobin treatment brought significant decreases in the NAG index and alpha 1M index, suggesting a protective effect on renal function (Study 2).