Alpha2-macroglobulin (AMG) concentrations have been measured in amniotic fluids from 33 pregnancies where the outcome was an infant with a neural tube defect. AMG ranged from 1.3 to 50 mug/ml in these samples, but was undetectable (is less than 1 mug/ml) in matched controls. Since the abnormal samples included four cases of spina bifida and eight cases of anencephaly before 22 weeks of pregnancy, measurement of AMG concentrations may be useful in the early antenatal diagnosis of neural tube defects. In some cases it gave clearer results than those obtained by measurement of amniotic fluid alphafetoprotein. However, care must be excercised to ensure that amniotic fluids are not contaminated by blood.