Retroviral vector-mediated gene transfer into human hematopoietic stem cells (HSC) may permit gene therapy of certain genetic diseases. Stimulation of HSC with hematopoietic growth factors (GF) has been shown to increase the level of retroviral transduction. We have studied the effect of basic fibroblast growth factor (bFGF), alone and in combination with other GFs, on the efficiency of transfer of the bacterial neomycin phosphotransferase (neoR) gene into human CD34(+)-enriched peripheral blood hematopoietic progenitor cells. The combination of bFGF, interleukin-3 (IL-3), IL-6, and stem cell factor (SCF) resulted in a transduction efficiency of 37 and 35% for G418-resistant colony-forming units-granulocyte/macrophage (CFU-GM) and mixed colonies multipotent colony-forming units (CFU-GEMM), respectively, which was significantly higher than the corresponding figures obtained with IL-3, IL-6, and SCF. The optimal concentration of bFGF was between 20 and 200 ng/mL. bFGF alone had no effect on the transduction rate. These results indicated a synergism in the action of bFGF, IL-3, IL-6, and SCF to enhance gene transduction rates into human hematopoietic progenitor cells.