Influenza A/PR/8/34 titre was determined by plaque assay on MDCK cells. PBS-washed MDCK were inoculated with 500 μl of virus dilution for 1-2h at 37°C. Cells were covered with 2 ml of MEM medium containing 1% purified agar (Oxoid, England) and 1-2 μg/ml trypsin (Sigma). After three days incubation at 37°C, plates were stained with 0.03% neutral red staining to facilitate plaque counting.