As representatives of the current circulating strains, we used A/ New Caledonia/20/99 (H1N1) (NC20) as the susceptible virus (susceptible to amantadine, ribavirin, and oseltamivir carboxylate), CA10 as the amantadine-resistant virus, and MS H274Y as the oseltamivir-resistant virus. Concentration-response curves for amantadine, ribavirin, oseltamivir carboxylate, and TCAD were generated against all three viruses, and the EC 50 s and IQs are summarized in Table 4 . Amantadine was effective at inhibiting NC20 and MS H274Y in vitro, resulting in IQs of 1.95 and 7.17, respectively. However, the IQ for amantadine was reduced to 0.02 when tested against CA10, indicative that an in vitro concentration representing the achievable plasma concentration at the recommended dose was not adequate to inhibit the amantadineresistant virus in vitro. An IQ of 0.02 represents a 100-fold reduction compared to NC20 and a 350-fold reduction compared to MS H274Y. Similarly, oseltamivir carboxylate was effective at inhibiting NC20 and CA10 in vitro resulting in IQs of 1.5 and 5.0, respectively, but not MS H274Y (IQ = 0.004). Similar to amantadine against the amantadine-resistant virus, the IQ for oseltamivir was reduced 350-to 2300-fold against the oseltamivirresistant virus compared to susceptible viruses. The IQs for ribavirin were uniformly low and below 1 for all three viruses (0.23 to 0.42). On the other hand, the IQs for the TCAD regimen as a fixed dose combination were consistently high against all three viruses (8.33 to 17.24), varying by no more than 2-fold between susceptible and resistant viruses. These data suggest that the TCAD regimen may have broad utility against all circulating influenza strains, including strains that are resistant to either amantadine or oseltamivir.