The most common cause of cystic fibrosis is deletion of Phe-508 (delta F508) from the cystic fibrosis transmembrane conductance regulator (CFTR). Previous studies have suggested that delta F508 CFTR is an unstable protein that retains a pattern of glycosylation specific to the endoplasmic reticulum. This report examines the mechanism responsible for the mislocalization of delta F508 CFTR in a human cystic fibrosis epithelial cell line overexpressing recombinant CFTR by virtue of adenovirus-mediated gene transfer. Immunoelectron microscopy confirmed that wild-type CFTR is delivered to the plasma membrane of these cells and that delta F508 CFTR is retained in the endoplasmic reticulum. Pulse-chase studies showed that newly synthesized CFTR complexes with the chaperone hsp70. The wild-type protein dissociates from hsp70 before its transport to the Golgi, and the protein is subsequently degraded in lysosomes. By contrast, the complex formed between delta F508 CFTR and hsp70 is retained in the endoplasmic reticulum and delta F508 CFTR is rapidly degraded in a pre-Golgi nonlysosomal compartment. Thus, hsp70 discriminates between the normal form of CFTR and the form of the protein that most commonly causes cystic fibrosis (delta F508). These findings clarify the mechanism by which mutation causing delta F508 affects the intracellular trafficking of CFTR and suggest another function for hsp70 in ensuring quality control during the biosynthesis of plasma-membrane proteins.