A strain of high-altitude chickens (Gallus gallus) with a high hemoglobin-oxygen affinity has been found in the Peruvian Andes. The purpose in this work was to investigate the relation of this high affinity with the allosteric regulation of the inositol hexaphosphate effector (IHP). The hemoglobin-oxygen affinity has been determined in phosphate-free hemoglobin solutions before and after the addition of IHP. The results showed that the addition of IHP to the phosphate-free hemoglobin solutions increased the affinity significantly less in the high-altitude chicken (26.3 Torr) than in the sea-level chicken (42.2 Torr) (P < 0.001). It was concluded that the high-affinity hemoglobin is probably dependent on a molecular change affecting the allosteric regulation, although a change in the intrinsic properties of the hemoglobin cannot be discarded. © 1994.