ExtractIntrauterine exchange transfusions were conducted in fetal sheep using adult sheep as donors. Indwelling catheters in the fetus permitted sampling of blood for 8 to 36 days following the transfusion. In the first days after the procedure there was a decreased oxygen affinity of umbilical blood, an increase of about 5 mm Hg in umbilical venous O2tension, and a decrease of about 30% in O2saturation. In the following weeks there was a gradual return of O2affinity, PO2, and oxygen saturation to normal. In all of the experiments there was a good correlation between the O2affinity of umbilical blood and the percentage of adult cells present. This correlation indicated that adult cells retained a normal O2dissociation curve even after weeks of exposure to the new environment. Despite an increased umbilical venous PO2, the fetal reticulocyte count increased significantly after transfusion, but the oxygen capacity of umbilical blood remained within normal limits.SpeculationThese studies suggest that the correction of fetal anemia in erythroblastosis fetalis by the use of fetal blood instead of adult blood would produce a greater increase in the amount of hemoglobin capable of carrying oxygen at the low oxygen tensions characteristic of the fetal circulation.