Although cystic fibrosis (CF) is among the most common inherited diseases in Caucasian populations, the basic biochemical defect is not yet known. CF is inherited as an autosomal recessive trait apparently due to mutations in a single gene, whence the efforts made to identify the genetic locus responsible by linkage studies. Two markers have recently been identified that are genetically linked to CF: one is a genetic variation in serum level of activity of the enzyme paraoxonase, and the other is a restriction fragment length polymorphism (RFLP) identified with a randomly isolated DNA probe. We report here that the genetic locus DOCRI-917 defined by the cloned DNA probe is located on chromosome 7.