Amantadine (1-aminoadamantane hydrochloride) is effective in the prophylaxis and treatment of influenza A infections. In tissue culture this selective, strain-specific antiviral activity occurs at relatively low concentrations (5 microM or less), which inhibit either the initiation of infection or virus assembly. The data reported here demonstrate that the basis of these actions is similar and resides in the virus-coded M2 membrane protein, the product of a spliced transcript of RNA segment 7. Mutations which confer resistance to amantadine are restricted to four amino acids within a hydrophobic sequence, indicating that the drug is targetted against the putative membrane-associated portion of the molecule. The influence of the virus haemagglutinin on the amantadine sensitivity of virus strains implies that the drug may interfere with interactions between these two virus proteins.