For chronic neutropenic patients requiring long-term injection of recombinant human granulocyte colony-stimulating factor (rhG-CSF), a cellular transplantation system that can produce this cytokine stably and deliver it in a regulatory manner would be advantageous. In this study we aimed at developing a regulation system at cellular level using suicide vectors. We introduced the herpes simplex virus type 1 thymidine kinase (HSV-TK) gene into the rhG-CSF-producing NIH3T3 cells and examined if ganciclovir (GCV) treatment of the cells could control the rhG-CSF production in vitro. The cells transfected with the HSV-TK gene showed a > 100-fold increase in sensitivity to GCV compared with the parent cells, and the median inhibitory dose of GCV to the transfected cells was less than 1.6 microM. The total amount of rhG-CSF production by these cells was strongly suppressed by GCV treatment. This regulatory method may be applicable to cytokine supplement gene therapy.