A cDNA clone of the beta subunit of human fibronectin receptor (FNRB) detects two different polymorphic loci: (a) a codominant system previously mapped to the pericentromeric region of chromosome 10, the site of the functional FNRB gene; and (b) a dominant system not linked to the first one or to any chromosome 10 marker tested. This second polymorphism is characterized by the presence or absence of a band (or a set of bands). We have used linkage analysis and biotin-labeled in situ hybridization to map this dominant polymorphism to the short arm of chromosome 19; we hypothesize that it may be due to the insertion of part of the cDNA from the chromosome 10 gene into chromosome 19. This "insertion" is polymorphic in all populations studied.