SummaryRed cell glycolytic intermediates and enzymes in term infants in the first year of life were correlated with the fetal hemoglobin concentration (%F), intra- and extracellular venous pH, plasma inorganic phosphorus (Pi) and pyruvate kinase (PK) activity. Changes in the non-age-dependent enzymes phosphoglycerate kinase, enolase, and phosphofructokinase correlated most significantly with the postnatal decline in %F (P< 0.001), not the age of the red cell population, as reflected in PK activity. The age-dependent enzymes, hexokinase and glucose-6-phosphate dehydrogenase, however, correlated well with PK activity (P< 0.001). The concentration of glucose-6-phosphate did not correlate significantly with the postnatal decline in %F (P> 0.05) or PK (P> 0.10), but correlated significantly with the plasma Piconcentration (P< 0.001). “Total triose phosphate” and 2,3-diphosphoglycerate did not correlate with Pi.It appears from these studies that an extracellular factor, Pi, alters the pattern of glycolytic intermediates in term infants and that the postnatal changes in phosphoglycerate kinase, enolase, and phosphofructokinase are unique to the “fetal” red cell and reflect passage from fetal to “adult” erythropoiesis.SpeculationIt is proposed that the rise in the glucose-6-phosphate concentration in red cells from term infants previously reported is secondary to a combination of stimulation of hexokinase activity by plasma inorganic phosphorus and a relative block in glycolysis at the phosphofructokinase (PFK) step secondary to both decreased enzyme activity and decreased activation of PFK by plasma inorganic phosphorus. It is speculated from these studies that the relative block in glycolysis at the PFK step previously described in term infants is probably greater at thein vivolevel than that predicted from enzyme activity under optimalin vitroconditions.