Five DNA probes known to originate from the region 7q22-q31 were sublocalized by in situ hybridization to metaphase preparations of fibroblasts having besides a normal chromosome 7, a homologue 7 with an apparent interstitial deletion of a large part of band q22. A flow cytometric chromosome analysis confirmed a loss of material from one of the homologues of chromosome 7. Four of the probes, B79a, 7C22, metH, and pJ3.11, have been shown to be closely linked to the cystic fibrosis (CF) locus. We localized probes B79a and 7C22 to the part of 7q22 involved in the deletion, whereas metH and pJ3.11 could be assigned to band 7q31. Probe pJu28, for which polymorphisms have not yet been described, also appeared to derive from the latter band. Since pJ3.11 and metH are most tightly linked to the CF locus, this disease locus is indirectly assigned to 7q31. A comparison of our findings with linkage data suggests a discrepancy between genetic and physical distances in the region 7q22-q31.