Recombinant adenovirus has recently become a promising gene delivery vehicle that may be used therapeutically for various medical disorders. However, in vivo expression of transgenes delivered by E1 region-deleted adenoviral vectors is transient in immunocompetent animals. It has been proposed that destruction of adenovirally transduced cells by the host immune mechanisms, particularly cytotoxic T-lymphocytes, may play a major role in limiting the duration of transgene expression in vivo. In the present study, Southern blot analysis of genomic DNA prepared from transduced liver tissues showed the persistent presence of the viral genome in both immunocompetent and immunodeficient animals, indicating the survival of the adenovirally transduced liver cells. Furthermore, active expression of the surviving factor IX transgenes was shown by the presence of recombinant human factor IX as well as specific human factor IX mRNA and protein in the transduced liver tissues. The transient appearance of human factor IX in the circulation of normal as well as partially immunodeficient mice is primarily due to the generation of mouse antihuman factor IX antibodies in these mice rather than host immune destruction of transduced cells. These results suggest that liver cells transduced with recombinant adenoviral vectors can escape from being destroyed by the host immune mechanism in normal animals. The present study thus provides a new rationale for further engineering of adenoviral vectors into a durable expression system for gene therapy of various diseases including congenital disorders such as hemophilia B.