Heterogonous viral replication of pandemic influenza H1N1 strains in mice lungs. (a) Viral loads in the lungs of NC2, NC8 and NC11 infected C57/BL6 mice were determined at 1 d.p.i.. (b) Viral loads NC2 were also examined in C57/BL6 and BALB/C mice (n =3). Replication kinetics of NC2 and NC8 were compared in BALB/C mice infected with dose series of viruses at (c) 1 d.p.i. and (d) 3 d.p.i.. MDCK cells were used to determine viral loads in serially diluted lung homogenates collected at different time intervals. Results are presented as mean ± SEM of TCID50. Statistical differences were calculated by Mann Whitney U test. *** - P < 0.0001.