A radioimmunoassay for alpha-fetoprotein (AFP) is described and normal ranges for both maternal serum and in amniotic fluid throughout pregnancy are defined. Maternal serum AFP levels in at risk pregnancies were found to be no different from those in normal pregnancies. AFP levels in pregnancies complicated by neural tube and other congenital defects, fetal death or maternal hypertension are documented. Eight patients with a fetus deformed by anencephaly or an open spina bifida were tested before 22 weeks; seven of them had raised serum AFP levels. Other causes of raised serum AFP levels are described and the significance of a raised serum AFP level is discussed with particular reference to screening programmes.