The characteristic increased salinity of sweat and other abnormalities of exocrine secretions in patients with cystic fibrosis (CF) suggest the possibility of a disturbed functioning of the plasma membrane in this disease. Several lines of evidence indicate that fibroblasts express the presence of the CF genotype. Therefore these cells were used in an in vitro study directed at determining whether the manifestations of CF might be related to an alteration of one or more of the protein components of the plasma membrane. In order to evaluate the synthesis of these components, growing fibrosblasts from patients with CF and normal subjects were briefly exposed to either 14C- or 3-H-leucine. Their plasma membrances were then isolated and subjected to analysis in a nondetergent acrylamide gel system. Coelectrophoresis of differentially labeled preparations revealed the absence of a detectable abnormality in the synthetic rates of any of the more than 30 resolved protein species.