The correlation between red cell 2,3-DPG and serum levels of thyroid hormones (T4 and T3) has been evaluated in full-term newborns, 1 to 4 days old. A direct effect of thyroxine (T4) on the post-natal increase of red cell 2,3-DPG was excluded because no significant correlation was found between the rise of 2,3-DPG concentration and serum T4 level.