CYSTIC fibrosis is associated with a defect in epithelial chloride ion transport (reviewed in refs 1, 2) which is caused by mutations in a membrane protein called CFTR (cystic fibrosis transmembrane conductance regulator)³. Heterologous expression of CFTR produces cyclicAMP-sensitive Cl^--channel activity^4–7.Deletion of phenylalanine at amino-acid position 508 in CFTR (ΔF508 CFTR) is the most common mutation in cystic fibrosis⁸. It has been proposed that this mutation prevents glycoprotein maturation and its transport to its normal cellular location⁹. We have expressed both CFTR and ΔF508 CFTR in Vero cells using recombinant vaccinia virus. Although far less ΔF508 CFTR reached the plasma membrane than normal CFTR, sufficient ΔF508 CFTR was expressed at the plasma membrane to permit functional analysis. ΔF508 CFTR expression induced a reduced activity of the cAMP-activated Cl^- channel, with conductance, anion selectivity and open-time kinetics similar to those of CFTR, but with much greater closed times, resulting in a large decrease of open probability. The ΔF508 mutation thus seems to have two major consequences, an abnormal translocation of the CFTR protein which limits membrane insertion, and an abnormal function in mediating Cl^- transport.