The cystic fibrosis transmembrane conductance regulator (CFTR) is a phosphorylation-regulated Ch⁻ channel. In most mammalian cells, the functional consequences of the most common CF mutation, ΔF508-CFTR, cannot be assessed as the mutant protein undergoes biosynthetic arrest. However, function can be studied in the baculovirus-insect cell expression system where ΔF508-CFTR does not appear to undergo such arrest. Our results show that phosphorylation-regulated Cl⁻ channel activity of ΔF508-CFTR is similar to that of wild-type CFTR. This observation was confirmed in comparative studies of purified ΔF508-CFTR and CFTR reconstituted in planar lipid bilayers. Therefore, we suggest that this Common mutation does not result in a significant alteration in CFTR function