A colorimetric method for the determination of free haemoglobin in human serum is described, using 4-aminophenazone as chromogen, and commercially available haemiglobin cyanide solutions as standards. The reaction system is suitable for manual measurements as well as determinations with a centrifugal analyser (Cobas Bio System). Accuracy, linearity and stability of the procedure is shown. A reference interval in sera of normal adults was established, with an upper limit of 5 mg/dl. The alternative use of heparin plasma instead of serum is discussed.