Human umbilical cord blood (UCB) can be a source of hematopoietic stem cells for gene therapy, as an alternative to allogeneic bone marrow transplantation, for the treatment of a number of genetic diseases. To determine conditions that yield maximal gene transfer into UCB progenitor cells, we examined a number of variables. We used cell-free retroviral vector supernatants that convey neomycin (G418) resistance and measured the percentage of G418-resistant progenitor-derived colonies. Adding retroviral supernatant to the UCB cells in basal medium once a day for 3 days produced a threefold increase of G418-resistant colonies (9.8%) compared to a single exposure to supernatant (3.1%). To establish whether recombinant human growth factors are beneficial during transduction, the presence of interleukin-3 (IL-3), IL-6, and mast cell growth factor (MGF, a c-kit ligand) were compared in different combinations. Inclusion of the three factors together caused a threefold increase of gene transfer (30.4%) compared to transduction in basal medium. When the UCB cells were precultured in medium containing IL-3, IL-6, and MGF for 3 days before addition of the retroviral supernatant on days 4, 5, and 6, the average extent of gene transfer was 21.8%, compared with an average of 34.4% when UCB cells were transduced on days 1, 2, and 3. The presence of marrow stroma during the transduction of the UCB cells did not further increase gene transfer. We conclude that UCB progenitor cells can be efficiently transduced with the use of recombinant human growth factors IL-3, IL-6, and MGF and may be a suitable source for gene therapy.