We measured insulin-like growth factor II and nonsuppressible insulin-like activity levels in the sera of newborn infants with different birth weights and gestational ages to determine the significance of these peptides in fetal growth. Our results obtained by use of one-way analysis of variance showed that the insulin-like growth factor II and nonsuppressible insulin-like activity levels in premature, average-weight-for-gestational-age, large-for-gestational-age, and small-for-gestational-age newborns were significantly different (p less than 0.01). Although levels in the premature neonates were less than the other three groups and large-for-gestational-age neonates had a higher insulin-like growth factor II level than the other three groups, maternal insulin-like growth factor II levels in all groups were similar. These results suggest that insulin-like growth factor II may play a major role in fetal growth.