Madin-Darby-canine kidney (MDCK) cells were maintained in Dulbecco modifi ed Eagle medium (DMEM) containing 10% fetal calf serum (FCS), and 2 mmol/L Lglutamine, at 37°C in a 5% CO 2 humidifi ed atmosphere. The infl uenza (H5N1) virus used was strain A/Hong Kong/156/97 (kindly provided by Paul Chan) (15) . The virus stock used as challenge antigen in the hemagglutination inhibition (HI) assay was propagated in the allantoic cavities of 10-day-old embryonated hen eggs. The allantoic fl uid was harvested 48 h postinoculation and clarifi ed by centrifugation. Virus concentration was determined by HA titration as previously described (16) , and the virus was stored at -80°C until used. The virus stock used in the microneutralization (NT) and in the cell-mediated immunity assays was propagated in MDCK cells, and the culture supernatants were collected 48 h postinoculation. The 50% tissue culture infectious dose (TCID 50 ), determined by titration in MDCK cells, was calculated by the Reed and Muench method (17) .