Using a monoclonal antibody specific for human fibronectin (FN), we screened hybrid clones derived from the fusion of FN+ human fibroblasts, carrying a 11/X translocation, and FN-, HPRT- mouse cells for the production of this glycoprotein. Since no hybrid clone retaining the human der 11 chromosome was found to produce any human fibronectin, the segment of chromosome 11 included in the rearranged chromosome (11qter leads to 11p13) probably does not carry the structural locus for fibronectin.