The red cell pH as a function of oxygen saturation of haemoglobin and the oxygen affinity of haemoglobin as a function of red cell pH in Lampetra fluviatilis were studied at intracellular pH values ranging from 6.7 to 8.0. The maximal increase in red cell pH upon deoxygenation, 0.326 units, was seen when the pH of oxygen-saturated erythrocytes was 7.7, a value corresponding to the red cell pH of resting lampreys. The effect of oxygen saturation on red cell pH was reduced both when the red cell pH was decreased and when it was increased. The haemoglobin-oxygen affinity was strongly pH-dependent, with a Bohr-factor of -1.03. At a high pH (7.67) maintained by lamprey red cells at physiological conditions, the P50 value was 28.2 Torr. At a pH value (7.33) similar to that of rainbow trout, the P50 value was 73 Torr. Thus, the high red cell pH in lamprey is required for effective oxygen loading in gills. It also maximizes the apparent cooperativity of oxygen binding.