Mutant (delta F508) and wild-type cystic fibrosis transmembrane conductance regulator (CFTR) were synthesized initially as an approximately 140-kDa core-glycosylated precursor, which, in the case of wild-type CFTR, was chased to an approximately 160 kDa form bearing complex oligosaccharides. Mutant CFTR disappeared from the detergent-soluble cell extract with rapid (t1/2 = 27 min) kinetics. Only approximately 25% of the initially synthesized wild-type 140-kDa CFTR precursor was detected as mature protein; the remaining approximately 75% decayed with kinetics (t1/2 = 33 min) indistinguishable from those of the mutant. Rapid degradation kinetics and inefficient processing of wild-type CFTR were also observed in the colonic carcinoma lines HT29 and T84 and in stably transfected C127 cells, which express 5-50 times lower levels of CFTR. These results suggest that inefficient processing and rapid degradation of wild-type CFTR precursor are an intrinsic property of CFTR in these diverse cell types and are not an artifact of overexpression. Degradation of wild-type and mutant 140-kDa CFTR began without significant lag following synthesis. These data suggest that wild-type and delta F508 CFTR differ in the efficiency of folding of the core-glycosylated primary translation product.