Plasma and red blood cell folate concentrations (L. casei-activity) have been studied at term in 166 infants born after normal pregnancies, and in 139 of their mothers. Iron (but not folic acid) supplementation was given during pregnancy. The mothers did not develop folate deficiency during pregnancy as judged from their plasma and red blood cell folate concentrations and peripheral red blood cell pictures. A positive correlation was observed between the red cell folate concentrations in the mothers and the birth weights of the infants (r = 0.18, n = 136, p less than 0.05). A positive correlation was also observed between the red cell folate concentrations of the infants and the birth weights and lengths (r = 0.16, n = 147, p less than 0.05 and r = 0.22, n = 147, p less than 0.01, respectively). The present evidence is insufficient to settle whether these correlations are causal. Hematological parameters in the mothers were correlated to those of the infants. The results suggest a relationship between the maternal and infant erythropoiesis.