Antisera against two peptides, corresponding to different domains of the cystic fibrosis gene product CFTR, have been raised and extensively characterized. Both antisera recognize CFTR as a 165-kDa polypeptide in Western analysis of cells transfected with CFTR cDNA as well as in epithelial cell lines. The cell and tissue distribution of CFTR has been studied by immunocytochemistry. CFTR is abundant in epithelial cells, including those lining sweat ducts, small pancreatic ducts, and intestinal crypts. Unexpectedly, the level of CFTR in lung epithelia is relatively low, while it is abundant in the epithelia of kidney tubules. The protein appears to be restricted to the apical, rather than basolateral, regions of epithelial cells and at least a proportion is associated with the plasma membrane. The cell and tissue distributions of CFTR are consistent with a function for this protein as a chloride channel or as a regulator of channel activity.