The recent identification of the cystic fibrosis (CF) gene and its putative protein product, the CF transmembrane conductance regulator (CFTR), enabled us to synthesize oligopeptides corresponding with a predicted extracellular domain (position 103-117; peptide A) and a cytoplasmic domain (position 501-515; peptide B) constituting the phenylalanine deletion (F 508) observed in the majority of CF mutations. Immunobiochemical studies with antibodies directed against these peptides revealed the presence of two CFTR candidate proteins (155 and 195 kDa) in various types of epithelial cells. Immunolocalization studies performed on slices of human duodenum showed the strongest expression in the endoplasmic reticulum (RER) of the mucus-producing Goblet cells. Labeling is also demonstrated in the RER and apical membranes of villus and crypt cells, however, to a weaker extent.