Insulin-like growth factor-I (IGF-I) is a polypeptide that mediates growth-promoting actions of GH and has insulin-like activity. Increase in the levels of IGF-I in the maternal and fetal circulation during pregnancy and the identification of the specific receptors for IGF-I in a variety of fetal tissues suggest that IGF-I may play a significant role in the regulation of fetal growth. The present study was undertaken to determine the significance of maternal IGF-I on fetal and placental growth by administering the specific antiserum for IGF-I to pregnant mice. The antiserum was produced by repeatedly injecting recombinant IGF-I into rabbits. The antiserum obtained showed 30% binding to 125I-IGF-I at a dilution of 1:240,000, and crossreactivity with IGF-II was 0.012%. In the first set of experiments, pregnant mice were given a daily dose of 50 microliters of the original antiserum (E-1) or normal rabbit serum (C-1) into the peritoneal cavity between Day 3 and 10. In the second set of experiments, antiserum (E-2) or normal rabbit serum (C-2) was administered between Day 11 and 18. On Day 18, all mice were killed, blood was collected for measurement of levels of IGF-I by RIA, and the weights of fetuses and placentas in individual mice were recorded. The maternal levels of IGF-I in group E-1 and E-2 were extremely low compared to those in group C-1 and C-2. The number of fetuses and rate of abortion among each group were not significant.(ABSTRACT TRUNCATED AT 250 WORDS)