Vaccine development based on a cell culture system has advantages over egg-based technology because H5 strains are highly pathogenic for chickens and supplying large numbers of embryonated eggs could be diffi cult in a pandemic. In addition, potential allergic reactions to egg components would be avoided by growing the vaccine virus in tissue culture cells. Recently, mammalian cell culture was used for propagating viruses to prepare killed infl uenza vaccine (28) . Inactivated infl uenza vaccines produced with Madin-Darby canine kidney (MDCK) and Vero cells, which served as vaccine substrates, have been licensed in the Netherlands. Of note, the human cell line PER.C6 may provide a useful cell-based system because, unlike MDCK and Vero cell systems, it does not require a solid matrix support for the growth of cells. Selecting background viruses that grow well in these cell cultures and monitoring them for antigenic changes and contaminating microbes during propagation of the virus in cell culture need to be considered.