Gene therapy vectors based on murine retroviruses are unable to transduce non-dividing cells. This has proven a particular problem in the haematopoietic system where the target cells of choice, the pluripotent stem cells are quiescent. In an attempt to circumvent this difficulty we have constructed a retroviral producer line that expresses the membrane bound form of human recombinant stem cell factor (SCF) on its cell surface. This should enable the retroviral producers to deliver a growth signal to the target cells simultaneous with their exposure to retrovirus. We tested the ability of these modified producers to transduce a growth factor-starved, SCF-dependent cell line (TF-1) and demon- strated that these cells, though quiescent, can still be successfully transduced. This approach was extended to targeting of umbilical cord blood CD34+ cells, a predominantly quiescent population that normally require the addition of cytokines for efficient transduction. Using the SCF-expressing producer line in the absence of exogenously added cytokines, we observed a marked stimulation in transduction efficiency over that achieved using the parent producer line alone. Colonies derived from these cells arising in semi-solid media were also shown to be positive for expression of a retrovirally encoded reporter gene.