1. The position of the oxygen-haemoglobin dissociation curve has been related to erythrocyte 2,3-diphosphoglycerate (2,3-DPG) concentrations in twenty-seven patients with both acute and chronic respiratory failure.
2. The average values of both 2,3-DPG concentration and of the oxygen tension for one-half full saturation (P50) in these patients were normal, but values both above and below the normal range were encountered.
3. P50 values correlated positively with erythrocyte 2,3-DPG concentrations, but negatively correlated with the mean corpuscular haemoglobin concentration in these patients.
4. Multiple regression, including the above two variables and also the blood oxygen capacity as major factors, accounted for just under half of the variance in the position of the dissociation curve, and this was little improved by addition of the minor effects of the arterial oxygen and carbon dioxide tensions.
5. The intraerythrocytic pH bore a similar relationship to plasma pH, in both studies in vitro of blood from normal subjects and patients with chronic hypercapnia, and studies in vivo in hypercapnic subjects.