The time course of the transition from HbF to HbA production in the postnatal period has been studied. The description is based on the time-dependent pattern of total haemoglobin and of HbF observed in an 8 months follow-up of 25 premature newborns. The absolute amount of HbF decreases exponentially from birth to approximately 25 weeks (from 15.56 +/- 0.48 g/dl to 0.66 +/- 0.08 g/dl), at a weekly rate of approximately 16%. On the other hand, HbA shows a biphasic pattern: first it decreases slightly from birth to 6 weeks (from 4.09 +/- 0.32 g/dl to 2. 63 +/- 0.33 g/dl); afterwards it increases exponentially at a weekly rate of 5% up to a plateau value, until HbF is almost completely replaced by HbA. Therefore the total haemoglobin decreases exponentially from birth to approximately constant for the next 6 weeks, and thereafter it increases very slowly. From these data, two distinct phases of postnatal erythropoiesis can be identified: the first (from birth to 6 weeks) is characterized by a decrease in the total amount of haemoglobin produced whereas in the second there is an alteration in the relative proportions of HbF and HbA being synthesized.