The neuraminidase inhibitor oseltamivir has been identified to have significant anti-influenza activity in clinical practice. However, its efficacy has not been verified in enough subtypes of influenza A virus, particularly, the current pandemic virus, H1N1. In vitro, using our influenza pseudotyped particle system, oseltamivir displayed significant inhibitory effects on vital NA activity and pp release. Conversely, a boosting effect on viral infection was observed, particularly with the 2009 H1N1 pp at oseltamivir concentrations above 0.025 mu M. Further testing on two wild 2009 H1N1 virus strains, A/California/07109 and A/Sichuan/1/09, as well as a seasonal flu virus, A/Baoan/51/2008, confirmed these findings. (C) 2009 Elsevier Inc. All rights reserved.