Lily, R. E. C., Cole, P. V., and Hawkins, L. H. (1972).Brit. J. industr. Med.,29, 454-457. Spectrophotometric measurement of carboxyhaemoglobin. An evaluation of the method of Commins and Lawther. The spectrophotometric method of Commins and Lawther for the measurement of carboxyhaemoglobin (COHb) concentration was evaluated for both accuracy and sensitivity. The method was calibrated by equilibrating diluted blood samples with known concentrations of carbon monoxide (CO) in air and measuring the resultant COHb saturations. Over the range 0-40% COHb saturation the mean error was 0·67% COHb (SD 0·65). It is considered that the accuracy of the method is more than sufficient for measurement of both the high COHb levels found in clinical situations and the lower COHb saturation resulting from exposure to urban air and cigarette smoking.