Concentrated proteins in pancreatic juice could precipitate and plug pancreatic ducts, initiating pancreatic disease. In cystic fibrosis (CF), pancreatic fluid secretion is impaired due to defective anion transport and proteins are hyperconcentrated. To determine whether proteins in pancreatic secretions precipitate selectively or nonspecifically, duodenal secretions were obtained from subjects with and without cystic fibrosis, during pancreatic stimulation with cholecystokinin (CCK) and secretin, dialyzed against phosphate-buffered saline (PBS), and concentrated in stages by ultrafiltration. Precipitates obtained by centrifugation at 15,600 g for 10 min at 4 degrees C were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The only protein band regularly enriched in precipitates from 10 CF and 9 non-CF samples had an Mr of approximately 14,000. Monoclonal antibodies, raised independently in two laboratories against either pancreatic stone protein (PSP) or pancreatic thread protein (PTP), reacted with the Mr 14,000 protein(s). Differential extraction of PTP and PSP by 0.1N HCl and 10% sodium citrate produced Mr 14,000 products that reacted equally with each monoclonal antibody (MAB). Two-dimensional gel electrophoresis demonstrated two Mr 14,000 spots in each extract with pI's of 5.8 and 6.0. Each spot reacted equivalently with the MABs for PSP and PTP. PSP/PTP type proteins are sparingly soluble in pancreatic secretions and could contribute to protein plugging in pancreatic disease.