Oxygen-binding, plasma and intra-erythrocytic pH, and haemoglobin, 2,3-diphosphoglycerate and inorganic phosphate concentrations were measured in sixty-two healthy non-smokers aged between 18 and 89 years. 2. P50 (oxygen tension at 50% oxygen saturation) expressed at plasma pH 7-40 and PCO2 5-33 kPa showed a positive correlation with age. 3. This correlation of P50 with age was closer when P50 was expressed at a constant intra-erythrocytic pH 7-20. On average P50 at intra-erythrocytic pH 7-20 increased from 3-59 kPa at 20 years to 3-96 kPa at 90 years of age. 4. 2,3-Diphosphoglycerate, inorganic phosphate, haemoglobin and mean corpuscular haemoglobin concentrations did not correlate with P50 or with age.
