The pmetH probe, tightly linked to the locus for cystic fibrosis (CF), detected a NotI polymorphism with allele sizes of 1,000 and 550 kb that could be separated by pulsed-field gel electrophoresis. Both alleles were found on chromosomes bearing either the normal or the CF allele. Preliminary data showed that this polymorphism was not in strong linkage disequilibrium with the CF locus. Preliminary large-scale restriction mapping of the fragment showed that the NotI polymorphic site is 200-370 kb distant from the MET locus; thus it defines a new polymorphic locus in the CF region. This locus is of potential interest for studying CF families that show recombinants between CF and its tightly linked probes, in order to increase the precision of the genetic map of the region.