Second-trimester amniotic fluid contains two major alkaline phosphatase (ALP) isoenzymes, one susceptible to inhibition by phenylalanine and the other to inhibition by homoarginine. The proportions of these isoenzymes are constant between 15 and 21 weeks of gestation. In pregnancies where the fetus has cystic fibrosis there is a profound deficiency of the phenylalanine-inhibitable form which is thought to be derived mainly from mucosal cells of the fetal intestine. When phenylalanine and homoarginine were used to define ALP isoenzymes in stored amniotic fluids, 9 of 10 cases of cystic fibrosis were identified, while only 9 of 831 control fluids were scored as false positives. In pregnancies where there is an a priori risk of 1:3 (one or more previous affected children) a positive test indicates a 28:1 risk of cystic fibrosis in the fetus. This method is likely to prove satisfactory for the early prenatal diagnosis of cystic fibrosis.