In the present study we determined the antiviral effect of amantadine against influenza A/Netherlands/219/03 (H7N7) virus in cell culture and in a mouse model. Amantadine at concentrations <100 muM failed to inhibit virus replication in Madin-Darby canine kidney (MDCK) cells. When orally administered to mice for 5 days, amantadine at 15 mg kg(-1) day(-1) did not protect animals against lethal challenge with H7N7 infection, and virus titres in mouse organs were not reduced. However, sequence analysis of the M2 protein revealed none of the mutations previously described as being associated with amantadine resistance. We used reverse genetics to generate viruses containing the haemagglutinin (HA) or M gene of A/Netherlands/219/03 virus to investigate the role of these genes in amantadine sensitivity. All recombinant viruses carrying the HA segment of A/Netherlands/219/03 (H7N7) virus were amantadine-resistant, regardless of the origin of their other genes. To study the role of fusion activity in the mechanism of drug resistance, we introduced the Gly(23)-->Cys mutation in the H7 fusion peptide. This substitution resulted in a decrease of the pH of fusion and was also associated with reduced virus replication in both MDCK cells and mice, as compared to that of the wild-type virus. We suggest that H7 HA protein plays a role in amantadine resistance, although all HA amino acids that participate in drug resistance still remain to be characterized. Our finding reveals that sequence analysis of the transmembrane domain of M2 protein may not adequately identify all drug-resistant variants.