MDCK Madin-Darby canine kidney (ATCC CCL-34) and NIH 3T3 (ATCC CRL-1685) were maintained in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal calf serum and 2 mM L-Glutamine at 37°C in 5% CO 2 . Influenza A/Puerto RicoR/8/34 (H1N1, Mount Sinai strain) virus was generously provided by Stephan Ludwig (Virology, ZMBE, Muenster, Germany).