Somatomedin concentrations in human umbilical sera (n = 206) were measured using a specific radioimmunoassay for insulin-like growth factor (IGF)-I and a specific radioreceptor assay for IGF-II following acid-ethanol extraction of the sera to remove the somatomedin binding proteins. IGF-I concentrations were lower (P less than 0.001) than adult values and correlated with gestational age (P less than 0.001) and birth weight (P less than 0.0001). Multiple regression analysis demonstrated that both birth weight expressed independently of gestational age as the standard deviate score (P less than 0.0001) and gestational age (P less than 0.002) had effects on umbilical cord IGF-I concentrations. IGF-II concentrations were similar to adult values and did not correlate with gestational age, birth size or IGF-I values. IGF-II concentrations were higher (P less than 0.005) in male than female fetuses. These data support a role for IGF-I in influencing fetal growth and suggest the independent regulation of the secretion of IGF-I and II in the perinatal period. These was no evidence to suggest a distinct perinatal form of somatomedin.