It is well known that hemolysis by extracorporeal circulation is major cause of renal failure after open heart surgery. The purpose of this study is to investigate the effects of administration of haptoglobin (Hp) during extracorporeal circulation. The patients were divided into two groups: Group I 10 patients underwent open heart surgery for extracorporeal circulation; Group II 10 patients underwent open heart surgery for extracorporeal circulation with the administration of 4,000 IU of Hp. The serum level of total Hp was elevated in group II during bypass, and reduced at 1st day in both groups more than pre-operative level. The serum level of total Hb was elevated in both groups during bypass, and reduced at 1st day within normal limits. Free serum Hb was found in group I at 30 min after start bypass and increased during bypass and urinary Hb was also found. However, in group II free Hb was not found during and after bypass. Urinary NAG and alpha 1 Mg levels of group I were significantly higher than those of group II. The administration of Hp during extracorporeal circulation prevents the increment of serum free Hb and is effective for protection of renal function.