These hints prompted us to investigate the effect of M2 over-expression on the influenza A virus life cycle in MDCK cells. We hypothesized, that surplus M2 fusion protein may reduce the concentration of available, functional Cav-1 by complexing. To monitor M2 protein levels and localization we generated mammalian expression vectors containing cDNAs for fusion proteins of M2 (A/ PR/8/34) with desRedExpress, a red fluorescent, tetrameric protein (pM2PR8DsRed) or EGFP (enhanced green fluorescent protein) (pM2PR8_EGFP) and transfected purified DNA into MDCK cells. Expression levels and localization of the fluorescent proteins were followed 1 and 2d after transfection. The transfection efficiency (ratio of fluorescent/nonfluorescent cells) ranged between 10 and 15%. M2 fluorescent fusion proteins initially were found in the cytoplasma and started to localize at the plasma membrane at day 1 post transfection. As expected M2DsRed and M2EGFP localization did not differ from localization of M2 after infection with A/PR8/ (Fig. 4) . Interestingly, M2 expression in infected MDCK decreased viral titres to 40% (M2DsRed) and 85% of (M2EGFP) of the level of non-transfected cells. Thus, M2 over-expression interferes with human influenza virus propagation, presumably by competing with endogeneous M2 for Cav-1 interaction.