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The erythrocyte count of the adult ostrich ( NN 2.0 x 106/mm3) is about 40% of that of man (z 5.0 x 106/mm3) but the red blood cell is approximately three times larger (220 pm3). The total phosphate content per cm3 red blood cell is 37.7 pmol. Inositol tetrakisphosphate (inositol-P4), a compound not previously reported in the erythrocytes of any species of animal, is the major organic phosphate in erythrocytes of the adult ostrich, accounting for 35 % of the total phosphate. Inositol pentakisphosphate (inositol-Ps), the major phosphorylated compound in the erythrocytes of most mature species of birds, is also present but in amounts approximately half that of inositoLP4. The effects of adenosine triphosphate, 2,3-bisphosphoglyceric acid (Pz-glycerate), inositoLP4, inositolPs, and inositol hexakisphosphate (inositol-P6) on the oxygen affinity of whole 'stripped' hemoglobin, hemoglobin A (major component), and hemoglobin D (minor component) isolated from erythrocytes of the mature ostrich have been determined. All five compounds cause a right shift in the oxygen dissociation curve of each hemoglobin and their relative effectiveness in decreasing the oxygen affinity is ATP < P2-glycerate < inositol-P4 < inositol-P5 < inositol-P6. No preferential interaction of inositol-P4 or inositol-P5 with either hemoglobin A or hemoglobin D was observed. The unique organic phosphate composition of the erythrocyte of the ostrich probably explains the relatively high whole blood oxygen affinity (PSO = 24.5 torr) of these birds.Recent observations suggest that changes in the composition of the intracellular organic phosphates within the erythrocyte are primarily responsible for the decrease in whole blood oxygen affinity which occurs during development of the late embryo and newly hatched bird [I -51. The remarkable changes in cellular composition which occur include the accumulation of 2,3-bisphosphoglyceric acid (P2-glycerate) for several days prior to hatching and its abrupt elimination just after hatching, the sudden increase in adenosine triphosphate at the time of hatching which persists at high concentrations for about two weeks, and the more gradual accumulation of inositol pentakisphosphate (inositol-PS). This latter compound becomes the predominant organic phosphate of the red blood cell within 7-10 days after hatching. In most avian species inositol-Ps is present in high conThis is paper no. 6 in the series "Studies on Avian Erythrocyte Metabolism".Ahbreviations and Symbols. Inositol tetrakisphosphate, inositol-P4 ; inositol pentakisphosphate, inositol-PS ; inositol hexakisphosphaie, inositol-Ps ; 2,3-bisphosphoglyceric acid, Pz-glycerate, Pso, partial pressure of oxygen in mmHg at which half the hemoglobin is saturated (1 mmHg = 13.5951 x 980.665 x lo--' Pa).centrations in the erythrocytes of the adult and is thought to be the major factor responsible for the relatively high P50 of the whole blood in these animals. Both P2-glycerate and inositol-P5 lower the oxygen affinity of hemoglobin (giving a higher PSO) and presuma...

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