The experiment was performed on 90% confluent MDCK cells growing in T25 flasks (Nunc, Rochester, NY, USA) in MEM supplemented with 10% FBS. PN-SIA28 as well as e137 mock control, were diluted in 1.5 mL MEM supplemented with TPCK trypsin (2 mg/ml) to obtain final concentrations of 2 mg/ml, 10 mg/ml and 20 mg/ml of antibody. 100 TCID 50 of A/PR/8/34 were prepared in 1.5 mL of the same medium. The two solutions, containing antibodies and virus, were mixed in order to obtain a final concentration of 1 mg/ml, 5 mg/ml and 10 mg/ml for the mAbs in a final volume of 3 mL. The mixes were then incubated 1 h at 37uC. An infection positive control (virus without PN-SIA28) was included as well as non-infected cells. After two washes with sterile PBS 16 (Gibco Invitrogen, Carlsbad, CA, USA), 1 ml of each neutralization mix was added to the flasks containing MDCK cells and the infection was performed for 1 h at 34uC in 5% CO 2 . After absorption, the medium was removed and the monolayer washed twice with sterile PBS. 3 mL of MEM supplemented with trypsin (2 mg/ml) was added to the infection positive control and to non-infected cells. Antibody PN-SIA28 and e137 were added to the previously treated infected cells, maintaining the concentrations used during the infection step. Cells were incubated at 34uC in 5% CO 2 and regularly checked for 48 hours for the presence of cytopathic effect (CPE) by comparing the positive infection control to the treated infected cells. The supernatant was then centrifuged (2000 rcf for 10 minutes), collected and stored at 280uC. All the viral stocks were titrated and used to infect new cell preparations, increasing, when it was possible, the concentration of PN-SIA28. After ten passages, the cells of the infection positive control and mock control were compared with infected cells under selective pressure of PN-SIA28. Once a strong cytopathic effect was evident in the positive and mock controls, the presence or absence of CPE in the PN-SIA28 treated infected cells was evaluated. All the supernatants were collected, centrifuged, stored and used to perform a full length DNA sequence of influenza genomic fragment 4 coding for viral HA.