As an extension of these studies we assessed the contribution of amantadine to the synergy of the TCAD regimen against . As a single agent, amantadine had no activity against these viruses at concentrations up to the highest concentration tested (32 mg/mL; data not shown). As shown in Figure 2 , against NC V27A and WI S31N, amantadine contributed to the synergy of the TCAD regimen in a concentration-dependent manner. The synergy of the TCAD regimen in the presence of amantadine was greater than the synergy of the ribavirin/oseltamivir carboxylate double combination without amantadine, and the increase in the synergy of the TCAD regimen was observed at 0.32 mg/mL amantadine for NC V27A and 1 mg/mL amantadine for WI S31N, when significance was evaluated at the level of ,0.05. Against DK A30T, however, amantadine did not make a contribution to the synergy of the TCAD regimen within the concentration range tested (0.1-3.2 mg/mL), and the synergy of the TCAD regimen was not greater than the synergy of the ribavirin/oseltamivir carboxylate double combination. Whether the lack of contribution from amantadine against the DK A30T virus was due to the specific subtype (H5N1), the M2 mutation (A30T), or the combination of both remain to be determined. However, it should be noted that while amantadine made no contribution to the activity of the TCAD regimen against DK A30T, both ribavirin and oseltamivir remain active and their interactions were additive, indicative that two out of three drugs contribute to the activity of the TCAD regimen against this virus (data not shown).