293T cells transfected with H5 HA were detached with and resuspended in PBS, blocked in 10% horse serum and then labelled with a polyclonal rabbit anti-H5N1 antibody (Sino Biologicals Inc., Beijing, China) at a 1:400 dilution for 1 hr at 4uC. Unbound antibodies were removed by washing three times with cold PBS, followed by staining with a phycoerythrin (PE)-conjugated, donkey-anti-goat secondary antibody (Jackson Immunoresearch Laboratories, Suffolk, UK) for 30 min at 4uC. Data were collected from at least 5000 cells on an LSRII flow cytometer (BD Biosciences, Franklin Lakes, NJ, USA) and post-acquisition analyses of cell surface expression of H5-HA was performed using FlowJo software (TreeStar, Ashland, OR, USA).