We studied the effect of bafilomycin A1 (Baf-A1), a novel and highly specific inhibitor for vacuolar-type proton (V-H+) pump, on the growth of influenza A and B viruses in Madin-Darby canine kidney cells. Vital fluorescence microscopic study showed that Baf-A1 induced the complete disappearance of acidified compartments such as endosomes and lysosomes both in infected and uninfected cells by the treatment with 0.1 microM inhibitor for 1 h at 37 degrees C. In addition, virus growth was inhibited when Baf-A1 was present from 1 h before infection to the end of incubation, or added within as early as 5-10 min after infection. Conversely, the virus growth was recovered in correlation with the reappearance of acidified compartments after removal of Baf-A1. These data suggest that Baf-A1-sensitive V-H+ pumps are solely responsible for the acidification of endosomes and lysosomes, and thus Baf-A1 inhibits the growth of influenza A and B viruses by affecting the acidified compartments in which low pH is essential for the uncoating process of influenza virus growth at an early stage of infection.