The following reference strains were tested in the BLS3 laboratory of the Vita-Salute San Raffaele University: (H1N1 . The A/swine/Parma/1/97 isolate was analogously grown on NSK (Newborn Swine Kidney) cells, kindly provided by the Zooprophylactic Institute of Brescia, Italy. At 80% confluence, cells in MEM supplemented with 2 mg/ml serum-free TPCKtrypsin (Roche Applied Science), were infected with each strain at a MOI of 0.001. After 1 hour of infection, cells were washed with PBS (Phosphate buffered saline); MEM supplemented with 2 mg/ ml trypsin was then added and cells were incubated at 37uC in 5% CO 2 atmosphere. Cells were observed daily to monitor the cytopathic effect and, usually after 72-96 hours, the supernatant was collected, centrifuged at 1000 rcf for 10 minutes to eliminate cells debris and filtered with 0.22 mm filters (Millipore, Billerica, MA, USA). The supernatant was then aliquoted and stored at 280uC as cell-free virus stock.