A method is described for the measurement of the half saturation pressure (p50) and the "standardised O2-saturation", as an index of the oxygen affinity of haemoglobin. In addition to the normal blood gas analysis, the only other parameter required is the O2-saturation after equilibration of the blood at pO2 = 31 mm Hg (4.72 kPa). The results are read from a specially prepared nomogram. Potential uses and limitations of the method are discussed.