RB6-8C5 treatment during influenza infection exacerbates disease and leads to increased virus growth in the airways. Groups of 5 B6 mice were treated with mAb RB6-8C5 (RB6) to deplete neutrophils, or with either rat IgG (IgG) or diluent alone (PBS) 1 day prior to infection with 105 PFU of HKx31, and every second day thereafter as described in Materials and Methods. (A) Animals displaying evidence of pneumonia and/or having lost > 25% of their original body weights were euthanized. Survival curves were constructed using pooled data from 2 independent experiments (n = 10 mice/group) (B) Mice were weighed daily and results expressed as the mean percent weight change of each group + SEM, compared to the weight immediately prior to infection. (C) Growth and replication of influenza virus in the upper and lower respiratory tract. At various times post-infection, lungs and nasal tissues were removed and levels of infectious virus were assayed in clarified homogenates by plaque assay on MDCK cells. Data represents the mean virus titre ± 1 SD (n = 5 mice/group). ** indicates viral titres of RB6-8C5-treated mice significantly different to those of IgG-treated controls (p < 0.01, Student's t-test). PBS- and IgG-treated controls were not significantly different to each other at any time-point tested.