A new technique of measuring stool enzyme activity on dry specimens of faeces from newborn children at 4-5 days of age has detected 3 cases of cystic fibrosis in the first 6000 tests. No known cases of cystic fibrosis have been missed. Additionally, one case of pancreatic achylia of at least 4 months' duration has been detected. It is proposed that the detection of cystic fibrosis by this technique is sufficiently practical to be acceptable as a worthwhile newborn screening programme. The screening test has been in use in Auckland for over a year and is now being set up in Hamilton, Wellington, and Dunedin (New Zealand), and Sydney (Australia).