The question of the mechanism of the physiological anemia of childhood was reexamined; we took into consideration the fact that microcytosis is a feature of this anemia as well as the evidence suggesting that the 2,3 DPG/hemoglobin ratio may depend in part on red cell size. In a group of normal, not-iron deficient children a high inverse correlation was obtained between the MCV and the 2,3 DPG/Hb ratio; this latter ratio did not correlate with serum phosphorus levels, previously incriminated in the indirect causation of the anemia of childhood.