Specimens of meconium and random stools were collected sequentially from 25 healthy newborn babies over the first 8-14 days of life. The stool chymotrypsin concentrations increased from birth to a maximum at 4 days of age and then fell again over the next four days. The lowest individual stool concentrations either side of the four day peak were both, coincidentally, 120 micrograms/g stool. In a second group of 22 newborn babies suspected of meconium ileus and later confirmed to have cystic fibrosis, faecal chymotrypsin concentrations were all appreciably reduced. In eight babies, also with suspected meconium ileus but with negative sweat tests, chymotrypsin concentrations were within the healthy newborn range. Measuring faecal chymotrypsin concentrations is a reliable procedure for identifying pancreatic exocrine insufficiency in the newborn.