To elucidate the physiological significance of epidermal growth factor (EGF) in fetal and placental development, the effects of neutralization of circulating EGF by anti-EGF antiserum on fetal and placental development were evaluated in pregnant mice. Abdominally administrated antiserum neutralized EGF in fetal and maternal circulation. Fetal weight was not affected by anti-EGF antiserum, but placental weight was suppressed when antiserum was administrated in the latter half of pregnancy. There was no significant difference between control and antiserum injected mice in the affinity and capacity of EGF receptor in the placenta. The number and volume of lamellar bodies were significantly decreased in antiserum treated mice at Day 10-18 compared to control mice. These results suggest that EGF is involved in placental development, especially in late pregnancy. Furthermore, EGF in the circulation may contribute to enhancing fetal lung maturity rather than fetal growth.