Although the majority of infections by the pandemic influenza H1N1 (2009) virus is mild, a higher mortality occurs in young adults with no risk factors for complications. Some of these severe cases were infected by the virus with an aspartate to glycine substitution at 225 position (D225G, H3 numbering) in the hemagglutinin (HA). Previous studies with the highly virulent 1918 pandemic H1N1 virus suggested that such substitution was associated with a dual binding specificity of the virus for both alpha2,3- and alpha2,6-linked sialic acid receptors on host cells. Thus, the D225G mutant may cause more severe disease with its increased predilection for the lower respiratory tract, where the alpha2,3 sialic acid receptor is more prevalent, but this hypothesis has not been investigated. We obtained a mutant virus after four sequential passages in lungs of BALB/c mice with a wild-type pandemic influenza A H1N1 (2009) virus. One plaque purified mutant virus had a single non-synonymous D225G mutation in the HA gene. This mutant was more lethal to chick embryo and produced a viral load of about two log higher than that of the wild-type parental virus during the first 24 h. A pathogenicity test showed that the 50% lethal dose in mice (LD50) was reduced from over 2 x 10(6) plaque-forming units (PFU) with the parental virus to just 150 PFU with the mutant virus. The survival of mice challenged with the mutant virus was significantly decreased when compared with the parental virus (P < 0.0001). Significantly higher viral titers and elevated proinflammatory cytokines in lung homogenates of mice infected with the mutant virus were found, which were compatible with severe histopathological changes of pneumonitis. The only consistent mutation in the genomes of viral clones obtained from dying mice was D225G substitution.