PR8 NL09 M virus and PR8 avian M virus, along with six PR8-based viruses carrying avian-human chimeric M segments, were inoculated at a MOI of 5 PFU/cell onto A549 cells. Virus growth was measured by plaque titration. The effects on viral growth of amino acid changes in (A) both M1 and M2, (B) M1 only and (C) M2 only are shown. Single-cycle growth was assessed in at least three independent experiments, with three technical sample replicates per experiment. Graphed data show the means with SD for at least three experiments. Statistical significance was determined using repeated measures, two-way, multiple ANOVA on log-transformed values, with Bonferroni correction applied to account for comparison of a limited no of means. Experiments involving PR8 NL09 9mut or 7mut viruses as well as PR8 Av M 9mut or 7mut viruses were performed at the same time, but are displayed on two separate graphs with the same PR8 NL09 M and PR8 avian M control virus data, to improve clarity of presentation.