Long-term parenteral administration of human alpha-interferon (HuIFN-alpha) is effective in the treatment of several malignancies, including chronic myelocytic leukemia. In the present study, a model for fibroblast-mediated HuIFN-alpha gene therapy for the treatment of chronic myelocytic leukemia is described. Human IFN-alpha 5 complementary DNA was inserted into a bovine papilloma virus plasmid vector (BMGNeo) containing a neomycin resistance gene. The recombinant plasmid (BMGNeo-IFN) was transfected into NIH/3T3 fibroblasts by the calcium phosphate coprecipitation method, and stably transformed cells were isolated by G418 selection. A fibroblast clone secreting a large amount of HuIFN into the culture supernatant was selected by radioimmunoassay using anti-HuIFN-alpha monoclonal antibodies. Southern blot analysis revealed that the transformed cells contained approximately ten copies of the BMGNeo-IFN plasmid per cell, and Northern blot analysis demonstrated high expression of HuIFN-alpha mRNA in the cells. This fibroblast clone strongly suppressed proliferation of a HuIFN-alpha-sensitive chronic myelocytic leukemia cell line (KU812) during cocultivation in vitro. When the HuIFN-alpha-producing fibroblasts were implanted into nude mice bearing KU812 tumors by the subcutaneous diffusion chamber method, tumor growth in vivo was also significantly suppressed. This study suggests the clinical potential of fibroblast-mediated gene therapy in the future.