We investigated whether the oxygen affinity of lamprey haemoglobin decreases with increasing oxygen concentration at the high (10-25 mmoll(-1) monomeric) haemoglobin concentrations prevailing within the erythrocytes. The intracellular concentration of haemoglobin was experimentally adjusted by shrinking the cells osmotically: the osmolality of the equilibration medium was increased from approximately 250 mosmol kg(-1) by 90 mosmol kg(-1) to 340 mosmol kg(-1) or by 180 mosmol kg(-1) to 430 mosmol kg(-1) by adding sucrose in the medium. This increased the mean cellular haemoglobin concentration from 16.9+/-0.23 mmoll(-1) (monomeric haemoglobin) to 20.0+/-0.20 mmoll(-1) (monomeric haemoglobin) and to 23.0.0+/-0.36 mmoll(-1) (monomeric haemoglobin), respectively (means +/- S.E.M., N=35-40; all the samples from 7-8 different pools of blood at each osmolality combined). The oxygen equilibrium curves at each osmolality were determined by Tucker's method. An increase in haemoglobin concentration shifted the oxygen equilibrium curve to the right as indicated by the P-50 values, which were 4.26+/-0.07 kPa at the lowest, 4.64+/-0.13 kPa at the intermediate and 5.64+/-0.40 kPa (means +/- S.E.M., N=7-8) at the highest haemoglobin concentrations. The decrease in haemoglobin oxygen-affinity was attributed to the volume changes, since the intracellular pH did not decrease with increasing mean cellular haemoglobin concentration. Thus, the variations in red blood cell volume commonly observed during hypoxia may play a role in the regulation of haemoglobin function.