Oxygen content measurements determined optically by the Co-Oximeter correlated closely with those obtained using the manometric technique of Van Slyke. In those samples with a high content of carbon monoxide, the agreement between the two methods could be improved, by taking into consideration the concentration of carboxyhaemoglobin (COHb), since this plays no part in oxygen transport. The calculated values of the oxygen-binding capacity varied widely, with mean values less than the theoretical figure of 1.39 ml O2/g Hb. This difference was greater in those samples with a higher content of COHb. When the amount of haemoglobin combined with COHb was taken into consideration the mean corrected values approximated to the theoretical value of 1.39, thus supporting this as the mean value for the oxygen-binding capacity of haemoglobin, although the large individual variability persisted.