We have observed that the alkaline Bohr effect of washed human fetal erythrocytes was larger than in human adult intact red cells, in physiological conditions of pH, PCO2 and temperature. This was also observed independently of the presence of CO2 and of 2,3 diphosphoglycerate (fresh or stored erythrocytes). 2. Experiments performed in purified HbFII and HbA1 solutions and direct titration of protons released upon oxygenation confirmed the larger alkaline Bohr effect of fetal hemoglobin, at physiological ionic strength. 3. At low chloride concentration HbFII solutions had an alkaline Bohr effect identical to that measured in HbA1 solutions. 4. Titration of purified Hb solutions with increasing concentrations of NaCl evidenced a lower O2 linked chloride binding by HbFII and predominantly at acid pH. 5. It is concluded that the larger alkaline Bohr effect of fetal erythrocytes of HbFII solutions is related to a diminished acid Bohr effect, due to the lower affinity of HbFII for chloride anions. 6. The physiological interest of these results for placental O2 transfer (double Bohr effect) and O2 delivery to the foetus is discussed.
