Concentrations of chorionic somatomammotrophin (hCS) in maternal plasma at the time of delivery and in placental extracts were determined by radioimmunoassay and related to neonatal growth in 111 normal pregnancies. Mean maternal plasma hCS concentration was lower in association with small-for-dates infants, defined by either height or weight, than with well-grown preterm or term infants. Mean placental hCS concentration was not decreased in association with undergrown infants though placental hCS was correlated with maternal hCS (r=0 - 245).