Hemoglobin-ligand interaction was studied in maternal and fetal sheep blood as a function of oxygen saturation. pH was changed by varying CO2 concentration (CO2 Bohr effect) or by addition of NaOH or HCl at constant PCO2 (fixed acid Bohr effect). For maternal blood, CO2 Bohr factor was -0.41 at 50% oxygen saturation, increasing in magnitude at lower saturation and decreasing in magnitude at higher saturation. For fetal blood, CO2 Bohr factor was -0.45 at 50% oxygen saturation, unchanging at lower saturation and decreasing in magnitude at higher saturation. Fixed acid Bohr factor was relatively saturation independent with a value of -0.36 for fetal blood and -0.27 for maternal blood. The pH-independent effect of molecular CO2 on oxygen affinity was markedly saturation dependent being greatest at low oxygen saturation. The CO2 effect was greater in maternal blood than fetal blood. However, the magnitude of the saturation dependency of Bohr factor is not great enough to have major physiological significance in oxygen transfer across the sheep placenta.