Insulin hs been implicated as a growth factor in fetal life. To clarify the role of insulin in the fetus, we studied insulin receptors on mononuclear leukocytes in placental-cord blood from 12 normal newborns. Eight healthy young adults served as controls. Specific binding (mean +/- S.E.M.) in the absence of unlabeled insulin per 10(7) monocytes was, respectively, 24.3+/-3.5 and 4.7+/-0.9 per cent in newborns and adults. This increase in binding is due to a rise in number of receptor sites per cell (newborns, 44,600, and adults, 7100) as well as an increase in receptor affinity (Ke for newborns = 5.9 X 10(8) M(-1), and that for adults = 2.9 X 10(8) M(-1). This finding of markedly high concentrations of high-affinity receptors for insulin on fetal cells supports the importance of insulin in intrauterine growth and development.