Cystic fibrosis (CF) is a lethal genetic disorder inherited as an autosomal recessive at a frequency of about 1/2000 in Caucasian populations. A DNA marker genetically linked to CF was identified through a collaborative effort by random screening with a collection of RFLP markers on a set of CF families. The marker (CRI-L917) was mapped to chromosome 7. Construction of a genetic linkage map spanning the entire chromosome has led to the identification of a subset of 11 markers close to and flanking the CF locus. Using techniques of pulsed-field gel electrophoresis, which allow very large DNA fragments to be separated, we used seven probes to generate a long-range restriction map covering 12 million base pairs surrounding the CF locus. Information from the map is being used to isolate new probes closer to the CF gene. Methods being developed will allow candidate genes to be tested for their ability to correct defects in ion transport in cultured CF cells.