Summary: The mononuclear cells of newborn infants compared to those of normal adults contain diminished activity of pyruvate kinase (PK) (14.9 ± 1.2 vs. 22.3 ± 1.3 μmol/1010 cells/min) and phosphoglycerate kinase (PGK) (195 ± 10 vs. 415 ± 19). These deficiencies are accentuated in the cells of premature infants (4.9 ± 0.5 and 71 ± 15, respectively) and are accompanied by reduced activity of adenylate kinase (AK) (14.8 ± 1.9 vs. 33.6 ± 2.6). The activities of AK and PGK in adult and neonatal mononuclear cells are stimulated by phytohemagglutinin (PHA). Although the PK activity of adult cells is stimulated by PHA, the PK activity of neonatal cells is not. Mononuclear cell glycolysis is stimulated by PHA in both adults and neonates. The mean lactate production of neonatal cells is slightly lower than it is in adult cells (93 vs. 110 μmol/1010 cells/hr, unstimulated, and 151 vs. 181, stimulated), but the differences are not statistically significant. The ATP content of neonatal cells (3.8 ± 0.4 μmoll 1010 cells) is reduced compared to adult values (6.0 ± 0.3). Lactate production is directly correlated with PK activity in adult and neonatal mononuclear cells. Neonatal mononuclear cell PK activity increases to adult levels during the first year of life. Speculation: Glycolytic metabolism of neonatal mononuclear cells differs from that observed in adult cells. Pyruvate kinase may play a critical role in the energy metabolism of immunocytes. The observed increase in mononuclear cell PK activity to adult values in the middle of the first year of life may suggest a correlation of PK activity with maturation of monocyte function.