Activities of the microvillar enzymes gamma-glutamyltranspeptidase (GGTP), aminopeptidase M (APM), phosphodiesterase and maltase have been examined in second-trimester amniotic fluid as possible aids to the early prenatal diagnosis of cystic fibrosis (CF). The two peptidases, GGTP and APM, gave best results. If the fifth percentile of the normal range is used as an action line, the sensitivity of a positive test (low GGTP value) is 78% and the predictability 84%. At the tenth percentile the sensitivity is 100% and the predictability 77%. These approximate figures apply only to pregnancies where there has been a previous affected child. Until the primary protein defect in CF is discovered, this may prove an acceptable form of prenatal diagnosis to the high-risk mother.