Evidence is presented indicating that human chromosome 11 carries a structural locus for fibronectin (FN). A panel of cell hybrids of FN fiber-producing normal human diploid cells fused with mouse A9 cells were immunofluorescence stained with species-specific, affinity purified antibodies to human FN (AHFN); similar anti-mouse FN antibodies (AMFN); and the nonspecies-specific anti-FN antiserum (AFN) which was used in an earlier study of this same hybrid panel (Eun and Klinger, 1980). All hybrids that had the human 11 produced extracellular fibers that fluoresced brightly when reacted with the AHFN and AFN. All other human chromosomes could be eliminated as being related to FN production, suggesting that the 11 caries a structural FN locus. This agrees with our earlier study in which a nonspecies-specific antiserum was used and with the preliminary report of Smith et al. (1979), who used a different cell system. The apparent discrepancy between these results and those of Owerbach et al. (1978), who reported synteny of FN with the human No. 8, was partly resolved in that one of their FN positive hybrids was found to react with our AMFN but not with the AHFN, suggesting that some other as yet undefined mechanism may be operating in their hybrids which were made with mouse LM/TK- cells. The parental mouse A9 cells of our hybrids do not produce FN fibers, although a radioimmunoassay detects a small amount of mouse FN secreted into the medium. The fibers of some of our AHFN positive hybrids also react weakly with AMFN, but none react only with AMFN. This suggests that fibers produced by these hybrids bind some mouse as well as human FN. Why this is not so in all the producers is not clear.