To obtain single agent concentration-response curves, individual drugs were added to MDCK cells in 96-well microplates (8610 4 cells/well) using three wells for each concentration used. The compounds were added at the following concentrations: oseltamivir carboxylate, zanamivir, and peramivir at 0, 0.000032, 0.0001, 0.00032, 0.001, 0.0032, 0.01, 0.032, 0.1, 1.0, 10.0 and 100 mg/mL; amantadine, rimantadine, and ribavirin at 0, 0.001, 0.0032, 0.01, 0.032, 0.1, 0.32, 1, 3.2, 10, 32 and 100 mg/mL. Untreated wells of infected cells (virus controls), uninfected cells (cell controls), and drug cytotoxicity controls (cells and drugs only, using the same dilution range for each drug as the test wells) were included on each test plate. At three days after infection, the virus control wells exhibited 100% cytopathology. The 50% effective concentration (EC 50 ) and 50% cytotoxic concentration (TC 50 ) was determined for each drug as outlined below.