Gene transfer to skeletal muscle was examined as a means of gene therapy for neutropenia. A recombinant retrovirus containing a human granulocyte colony-stimulating factor (G-CSF) gene was introduced into primary human or rat myoblasts, which were then shown to produce biologically active G-CSF. Transplantation of G-CSF-producing rat myoblasts into the muscle of syngeneic rats resulted in a 15-fold increase in absolute neutrophil counts. This increase correlated with detection of circulating human G-CSF protein throughout the 6-month duration of the experiment. These results clearly demonstrate long-term production of therapeutically relevant amounts of a human protein by normal cells in vivo.