The development of antibodies to a previously unexpressed protein product may limit the success of human gene therapy approaches. We inserted B-domain-deleted factor VIII (FVIII) cDNA of human, canine, or murine origin into the multiple cloning site of a liver-specific vector, pBS-HCRHPI-A, to yield plasmids pBS-HCRHPI-FVIIIA, pBS-HCRHPI-cFVIIIA, and pBS-HCRHPI-mFVIIIA, respectively. Fifty micrograms of each plasmid in 2 ml of solution was rapidly injected into the tail vein of three groups of hemophilia A mice. Factor VIII levels ranging from 3 to 12 IU/ml were obtained from all three groups (normal is 1 IU/ml in human plasma) 3 days after treatment. These initial very high levels of functional human, canine, or murine factor VIII, however, fell gradually to undetectable levels within 2–3 weeks, and their disappearance correlated with the generation of high-titer, inhibitory anti-FVIII antibodies. Notably, this immune response occurred independent of the species of origin of the exogenous factor VIII. Antibody titers to factor VIII were detected beginning at 2 weeks, reached a plateau and remained at high levels for over 6 months. The majority of anti-hFVIII IgG was IgG1 isotype specific, suggesting a humoral response mediated by Th2-induced signals. Consistent with this idea, in a separate group of mice treated with pBS-HCRHPI-FVIIIA, transient immunosuppression by cyclophosphamide significantly delayed (5/6) or abolished (1/6) inhibitory antibody formation against the transgene.