Human embryonic kidney (HEK) 293 cells, Madin-Darby canine kidney cells (MDCK), and mouse AB12 cells were maintained in Dulbecco's modified eagle's medium (DMEM), supplemented with 10% fetal bovine serum (FBS), L-glutamate, sodium pyruvate (NaPyr), and antibiotics. Fetal porcine retina cells (VR1BL E1), expressing human adenovirus 5 (AdHu5) E1a and porcine adenovirus 3 (PAV3) E1b large genes, were maintained in minimum essential medium (MEM) alpha, supplemented with 10% FBS, L-glutamate, NaPyr, non-essential amino acids, HEPES buffered saline, penicillin/streptomycin, and 50 mg/ml Hygro- mycin B (BD Biosciences). Avian influenza H5N1 strain A/ Hanoi/30408/2005 (H5N1) was generously provided by Q. Mai Le and T. Hien Nguyen, National Institute of Hygiene and Epidemiology, Hanoi, Vietnam. Virus was propagated on MDCK cells cultured with virus diluent (MEM, 0.3% bovine serum albumin, and antibiotics) containing 3.0 mg/ml TPCK-treated trypsin (TPCK-trypsin) and titered by plaque assay. All infectious recombinant adenovirus constructs were propagated on HEK 293 or VR1BL E1 cells and purified by a cesium chloride density gradient.