Human fetal hemoglobin is known to display, at 20 degrees C, a lower affinity than human adult hemoglobin for oxygen when both proteins are in the absence of organic phosphates. The physiologically important reverse situation is achieved at 37 degrees C upon addition of 2,3-bisphosphoglycerate (DPG), whose lower effect on fetal hemoglobin is related to some amino acid substitutions present in gamma-chains. However, the difference in oxygen affinity observed at 37 degrees C is not solely due to the different modulation power of DPG with respect to adult and fetal hemoglobins. In fact, the results presented here reveal new aspects linked to the interplay of temperature and organic phosphates. In particular, the lower effect of DPG on fetal hemoglobin renders almost identical the oxygen affinity of the two hemoglobins at 20 degrees C, abolishing the difference observed in the absence of the effector. Successively on going from 20 degrees C to 37 degrees C, by virtue of the lower overall heat of oxygenation (delta H) displayed by fetal hemoglobin when in the presence of DPG, adult hemoglobin shows a lower oxygen affinity, as it should if oxygen has to be transferred from maternal to fetal blood.