Serum alpha-fetoprotein (AFP) was measured in maternal, cord arterial and venous blood. Samples were collected at the time of vaginal delivery from 105 women at 36-42 weeks' gestation. There was a significant correlation between maternal, cord arterial and venous AFP. Umbilical cord arterial and venous AFP levels were considerably higher in male infants than in females. Umbilical AFP levels declined with lengthening gestation and increasing birthweight for both male and female infants and a similar pattern was seen in the mother. Fetal AFP levels were significantly higher in subjects giving birth at 40 weeks whose infants had a birthweight below the population mean vis-à-vis those above the mean. It is concluded that the absolute size of the fetus as well as gestational age may play a significant role in determining maternal and fetal AFP concentration.