In this study we tested whether the isoquinoline alkaloid berberine can inhibit the growth of influenza A. Our experiments showed strong inhibition of the growth of H1N1 influenza A strains PR/8/34 or WS/33 in RAW 264.7 macrophage-like cells, A549 human lung epithelial-derived cells and murine bone marrow derived macrophages, but not MDCK canine kidney cells. Studies of the mechanism underlying this effect suggest that berberine acts post-translationally to inhibit virus protein trafficking/maturation which in turn inhibits virus growth. Berberine was also evaluated for its ability to inhibit production of TNF-α and PGE(2) from A/PR/8/34 infected-RAW 264.7 cells. Our studies revealed strong inhibition of production of both mediators and suggest that this effect is distinct from the anti-viral effect. Finally, we asked whether berberine-containing ethanol extracts of goldenseal also inhibit the growth of influenza A and production of inflammatory mediators. We found strong effectiveness at high concentrations, although upon dilution extracts were somewhat less effective than purified berberine. Taken together, our results suggest that berberine may indeed be useful for the treatment of infections with influenza A.