Résumé : Mucoviscidosis, the most frequently lethal genetic syndrome of Caucasian population, is a recessive disease with multiple tissue involvement. Although the major pathological changes are observed in lungs and pancreas, abnormalities have also been detected in several other exocrine glands. For many reasons, such as the ready availability of tissue material, the absence of secondary changes and the potential for prenatal diagnosis, cultured skin fibroblasts could be the tissue of choice to search for the primary defect. Several abnormalities have been reported in CT fibroblasts, suggesting that the genetic abnormality is expressed in these cells. To search for potentially mutant protein(s) we have compared the protein composition of normal and CF fibroblasts by two dimensional gel electrophoresis and double-labeling autoradiography using 35S and 75Se methionine as tracer. The results demonstrate the power of the method; however, we have not found one protein spot consistently missing in CF cells. Possibly reasons for the absence of a single common identifyable defect are discussed.