It has been previously reported that green-tea extract (GTE) inhibits the growth of influenza virus by preventing its adsorption. In this study, we further investigated whether GTE exerts an additional inhibitory effect on the acidification of intracellular compartments such as endosomes and lysosomes (referred to as ELS) and thereby inhibits the growth of influenza A and B viruses in Madin-Darby canine kidney cells. The vital fluorescence microscopic study showed that GTE inhibited acidification of ELS in a concentration-dependent manner. Moreover, the growth of influenza A and B viruses was equally inhibited when the cells were treated with GTE within as early as 5 to 15 min after infection, depending on the virus strains. The fact that ( )epigallocatechin (EGC), one of major catechin molecules in GTE, exerts the inhibitory effects on the acidification of ELS and virus growth in a manner similar to that of GTE strongly suggests that EGC is one of the active components in the extract.