Reference values for postnatal serum alpha-fetoprotein (AFP) and concanavalin A (ConA) binding subfractions of AFP in preterm and term infants are presented. Preterm infants had 10-fold higher serum concentrations of AFP than did term infants at birth. The reduction of serum values of AFP was biphasic in both groups and differed significantly between the two groups. The first declining phase continued for approximately 4 months in preterm and for 2 months in term infants, and was related to the degree of prematurity. The AFP values reached adult levels by 12 months in preterm and by 9 months in term infants. The developmental pattern of the carbohydrate moiety of AFP was determined by ConA fractioning. The proportion of the ConA nonreactive subfraction of AFP in preterm and term infants at birth was 6% and 13%, respectively; it increased more rapidly in term than in preterm infants but reached 85% to 95% by the age of 6 months in both infant groups. Our results indicate that the postnatal maturation of AFP synthesis is dependent on gestational age. Malignant recurrences of neonatal sacrococcygeal teratomas were associated with an increase in serum concentration of AFP and a decrease in the proportion of the ConA nonreactive subfraction of AFP.