Acid mucins in goblet cells of the duodenum, jejunum and ileum of neonates and infants with cystic fibrosis (CF) and without CF were measured by scanning microdensitometry after alcian blue staining according to the protocol of McCarthy and Reid (1963) which characterises four groups of acidic mucins. In CF infants over 6 mth of age, but not in controls, there was an increase along the gut from duodenum to ileum of both weakly acidic and strongly acidic sulphomucins. In the ileum the increase was in total mucins from 6 mth previously in the same CF patients and this difference could be accounted for by an increase of sialidase-resistant mucins. The increase in sulphomucins was more marked at the tip than at the base of the villi. In CF neonates there was significant difference in the quantities of acidic mucins. The question whether the mucins of CF are chemically abnormal or merely accumulated to an abnormal extent is probably best investigated by analysis of sialidase-resistant mucins and sulphomucins of the ileum and strongly acidic sulphomucins of the duodenum and jejunum in CF infants over 6 mth of age.