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Abnormal retention of ΔF508 CFTR (cystic fibrosis transmembrane conductance regulator) in the endoplasmic reticulum is a major cause of cystic fibrosis (CF). We show that calnexin Δ185–520 but not calnexin can partially reverse the mislocalization of ΔF508 CFTR. This 256-amino acid protein has neither the transmembrane domain nor the P domain of calnexin. Calnexin Δ185–520 interacted with CFTR directly, and was secreted into the extracellular compartment over time. Forty-eight hours after transfection into CHO cells, calnexin Δ185–520 increased the conversion of immature ΔF508 CFTR into mature ΔF508 CFTR. In immortalized human CF cell lines expressing ΔF508 CFTR, a halide efflux assay showed that calnexin Δ185–520 partially restored CFTR function. These data indicate that calnexin Δ185–520 may give a clue to develop the therapeutic way of cystic fibrosis with ΔF508 CFTR.