1. The oxygen affinity in vitro, haematological indices, erythrocyte 2,3-diphosphoglycerate and plasma inorganic phosphate were determined in 20 patients with chronic ventilatory failure and in 20 healthy non-smokers of similar age.
2. No significant difference was observed between the mean oxygen affinity or phosphate concentrations of the patients and healthy subjects but the mean haemoglobin and packed cell volume were significantly higher in the patients.
3. There was a positive correlation between plasma and intraerythrocytic pH which was similar in both patients and healthy subjects.
4. The arteriovenous difference in oxygen saturation in vivo (directly measured at cardiac catheterization) correlated closely with that calculated from the individual patient's oxygen affinity determined in vitro and arterial and mixed venous oxygen and carbon dioxide tensions, suggesting that oxygen affinity in vitro accurately reflects the curve in vivo.