SummaryUsing anin vivoclosed-loop technique in rat jejunum, we have confirmed previousin vitrostudies that cystic fibrosis (CF) serum inhibits water, sodium, and glucose absorption and transmural potential difference. CF heterozygote serum or sera from children with diarrhoeal disorders not associated with exocrine pancreatic insufficiency had no effect on transport. Sera from patients with Shwachman's syndrome and exocrine pancreatic insufficiency had identical effects to CF serum. These results challenge the specificity of the CF “factor.”