To detect AH04 virus by fluorescent focus assay, infectious material from swabs was first allowed to amplify by inoculation into 1610 6 MDCK cells and incubation for 48 h. Supernatant virus was then diluted 1:10 in serum free DMEM and 250 ml used to inoculate 1.5610 5 MDCK cells in a 24 well tissue culture plate. After virus absorption, the cells were overlaid with 1 ml serum free DMEM media containing 1 mg/ml Worthington's trypsin and 0.14% BSA and incubated overnight at 37uC. The following day they were fixed with 4% formaldehyde in PBS, permeabilised by the addition of 0.2% Triton 6100 in PBS for 5 minutes at RT and fluorescently stained with anti-NP monoclonal antibody and counterstained for DNA with 4,6-diamino-2-phenylindole (DAPI) as previously described [17] . Cells were examined blind by two people and scored semi quantitatively for the presence of infected cells using a standardised schema (2 no fluorescence seen; +/2 some fluorescence seen (,5% cells infected); +5-10% cells infected; ++ .10% cells infected). The literature indicates immunofluorescence to be at least as sensitive in general as plaque assay [13, 18, 19] . Confirming this, tests using serial dilutions of known quantities of PR8 virus, our method reliably detected 20 PFU of virus in the original sample prior to amplification and 50% of the time detected 2 PFU (data not shown).