Double-copy retroviral vector containing human factor IX cDNA driven by human cytomegalovirus enhancer-promoter was constructed. The vector was introduced into the amphotropic packaging cell line PA317. The recombinant virus produced in PA317 was used to transduce skin fibroblasts from a hemophilia B patient. The infected cells produced high levels of biologically active human factor IX at a rate of 3420 ng/10(6) cells/24 h. These cells were embedded in a collagen matrix and implanted into the peritoneal cavity or subcutaneous space of mice. It was demonstrated that human factor IX was produced by the implants for at least 12 days in vivo, reaching a peak of 105 ng/ml plasma. Over 90% of the protein was functionally active. This technique has the potential to be developed into a new approach for gene therapy for hemophilia B.