The CO affinity of human fetal blood was compared with that of maternal blood. The influence of intraerythrocytic 2, 3-diphosphoglycerate(2, 3-DPG) concentration upon the CO affinity was also studied. Human fetal blood showed a lower affinity for CO than maternal blood at CO tension of less than 0.42 mmHg, while it had a higher CO affinity than maternal blood at CO tension of more than 4.13 mmHg. When CO tension was less than 0.42mmHg, fetal blood had less than 50% carboxyhemoglobin, and the difference in the CO affinity between fetal and maternal blood became more marked as the CO tension decreased. When the intraerythrocytic concentration of 2, 3-DPG was raised more than 10 times by incubating the stored, DPG-depleted blood with the mixture of inosine, pyruvate and phosphate, both fetal and maternal blood showed a decreased CO affinity, but the degree of the decrement was greater in the maternal blood than in the fetal blood. These results suggested than the intraerythrocytic 2, 3-DPG may be less responsible for the lower CO affinity at a given CO tension of fetal blood than for the relative increase in oxygen affinity of fetal blood.