The activities of two microvillar peptidases, gamma-glutamyl transpeptidase (GGTP) and aminopeptidase M (APM), have been measured in 132 samples of mid-trimester amniotic fluid. These included samples from 16 pregnancies at risk for cystic fibrosis. The activities of both peptidases were significantly below the normal range in amniotic fluids from the 6 affected pregnancies. This points to early pathological changes in fetal tissues in which microvilli are prominent. In contrast, 4-methylumbelliferylguanidinobenzoate-reactive protease activity in amniotic fluid from the 6 affected pregnancies was normal. Correlation of individual values between GGTP and APM was close in all cases examined, so that when a further 7 samples from cases at risk became available they were tested for GGTP alone. Of these, the 3 affected pregnancies had significantly reduced GGTP activity, particularly in the early weeks of gestation. It is suggested that early amniocentesis and examination of gamma-glutamyl-transpeptidase isoenzyme constitution might make possible the reliable early diagnosis of cystic fibrosis.