Hemophilia A is caused by blood coagulation factor VIII (FVIII) deficiency and is an attractive target for gene therapy. However, features of FVIII physiology, such as the instability of the mRNA and protein, have provided obstacles to the design of a feasible strategy for the transfer and expression of the human FVIII gene in vivo. We have constructed a recombinant adenoviral vector, Av1ALH81, that contains the human FVIII cDNA from which the B-domain has been deleted (BDD FVIII) and extensively characterized this vector in vitro and in vivo. In vitro, HepG2, human hepatoma cells, transduced with Av1ALH81 secreted high levels of biologically active human BDD FVIII measured by the Coatest bioassay (> 2,400 mU per 10(6) cells per 24 hr). Administration of Av1ALH81 to mice, via tail vein, resulted in expression of human BDD FVIII in the mouse plasma at levels averaging 307 +/- 93 ng/ml 1 week post-injection, measured by a sensitive human FVIII-specific ELISA. Normal FVIII levels in humans are 100-200 ng/ml, and therapeutic levels are as low as 10 ng/ml. Purification of the human FVIII from the mouse plasma, and subsequent Coatest analysis, revealed that the human FVIII produced in the mice was biologically active. In addition, the duration of FVIII expression in vivo was followed, and high-level FVIII expression was sustained over a period of several weeks. The finding that an adenoviral vector can mediate high-level expression of human FVIII in an animal model provides the basis for the development of gene therapy for hemophilia A.