Efficient transduction of reconstituting hematopoletic stem cells (HSC) is currently only possible by cocultivation of target cells directly on producer cell lines, a method not applicable to human gene therapy protocols. Our laboratory has previously shown adhesion of primitive hematopoletic stem and progenitor cells to the carboxy-terminal 30/35- kD fragment of the extracellular matrix molecule fibronectin (FN 30/35) (Nature 352:438, 1991) and increased transduction of human hematopoietic progenitor cells via retroviral vectors while adherent to this fragment (J Clin Invest 93:1451, 1994). Here we report that (1) transduction of reconstituting murine HSC assayed 12 months after infection with retrovirus supernatant on FN 30/35 is as effective as cocultivation directly on producer cells; (2) recombinant retrovirus particles directly adhere to FN 30/35 in a quantitative and dose- dependent fashion; and (3) increased transduction efficiency on FN 30/ 35 does not appear to be associated with increased cell proliferation or activation of protein phosphorylation typically induced by integrin- fibronectin interactions. Therefore, we speculate that supernatant infection of HSC on FN 30/35 leads to colocalization of retrovirus particles and target cells on FN 30/35 molecule with a large increase in local virus titer presented to the cell. These findings have direct and important implications for the modification of current human gene therapy protocols.