Hemoglobinemia is found in hemolytic anemia, paroxysmal nocturnal hemolobinuria and paroxysmal hemoglobinuria. Recently increasing use of extracorporeal circulation in cardiac surgery has stimulated the development of more precise methods for rapidly evaluating the formation and clearance of extracoporeal hemoglobin. A spectrophotometric method of analysis seemed to offer the best solution to the requirements. The method presented is on the basis of hemoglobin derivatives converted to cyanmethemoglobin, and the absorbance is measured in a spectrophotometer at 540 and 680 nm. The blank value ismeasured at 680 nm while the absorbance at 540 nm measures the hemoglobin derivatives. This method estimates all the hemoglobin derivatives such as oxyhemoglobin, carboxyhemoglobin, methemoglobin, sulfhemoglobin and hemialbumin. The method was tested in recovery experiments, which is given table 1. A good degree of correlation was obtained in a comparision with the method described by Crosby and Furth for non-hemolyzed serum. The spectrophotometric technique described offers many advantages in speed and simplicity over the chemical procedure.