Serum α-fetoprotein (AFP) levels were monitored in 32 normal babies consecutively from 2 to 3 days, 2 wk, and 2 and 4 months after birth. In addition, serum AFP concentration was also measured in 116 random specimens from infants with normal liver enzymes and 10 infants born immaturely. Results were combined to establish normal AFP levels for infants at various ages. Serum AFP disappeared rapidly after birth. We found that it was not until 8 months of age that the normal AFP level in infants approached adult level. The half-lives of AFP degradation were estimated to be 5.5 days between birth and 2 wk, 11 days between 2 wk to 2 months, and 33 days between 2 and 4 months of age. In contrast to earlier belief, we felt that some AFP synthesis still exists after birth; however, the rate of synthesis may also decrease with age.SpeculationOur results favor the view that some α-fetoprotein (AFP) synthesis exists after birth. The rate of synthesis is slowly decreased with time. Any factor affecting the rate of synthesis may influence the serum AFP level. The wide range of variation in serum AFP levels in infants of various ages indicates that there must be more than one unknown factor affecting the AFP serum level. The normal range will be narrower if these factors become known. By using our age-dependent normal AFP serum levels, it would be interesting to see whether differential diagnosis of neonatal hepatitis from biliary atresia can be unproved and whether infants do have higher and more frequent elevation of serum AFP with hepatic disorders.