The susceptibility of three 2009 H1N1 influenza strains -A/ California/04/09 (CA04), A/California/05/09 (CA05) and A/ California/10/09 (CA10) -to each of six antiviral drugs (amantadine, rimantadine, oseltamivir carboxylate, zanamivir, peramivir, and ribavirin) as single agents was determined by measuring the inhibition of virus-induced CPE in MDCK cells. Against the three strains, oseltamivir carboxylate, zanamivir, peramivir, and ribavirin produced concentration-dependent inhibition of cytopathic effect (data not shown). Amantadine was active only at higher concentrations (EC 50 of 16-20 mg/mL; 85-106 mM) (Table S2) , which represents a .250-fold reduction in susceptibility compared to the published values for a wild-type virus [17] . Rimantadine did not produce inhibition up to the 50% cytotoxic concentration (EC 50 .12 mg/mL; .55 mM). The EC 50 values for the six drugs as single agents against all three strains are summarized in Table S2 , and confirm that the three virus strains remain susceptible to oseltamivir carboxylate, zanamivir, peramivir, and ribavirin. These results are consistent with the results previously published that demonstrated that 2009 H1N1 contained a mutation (S31N) in the M2 channel that has been associated with resistance to adamantanes, but remained susceptible to NAIs [18] .