Semliki Forest virus (SFV) enters cells by receptor-mediated endocytosis, followed by acidification of endosomes by the action of the vacuolar H(+)-ATPase. Fusion of the viral and the endosomal membrane delivers the viral genome to the cytoplasm. Direct blockade of the vacuolar H(+)-ATPase by the selective inhibitor bafilomycin A1 (BFLA1) prevented the infection of cells by SFV, if the compound was present during the first minutes of infection. Attachment and penetration of virus particles were not the targets of the antibiotic. BFLA1 and the ionophore monensin potently blocked SFV infection even at low pH, indicating that acidic pH is not sufficient for SFV to deliver its genome to the cytoplasm, but the proper functioning of the H(+)-ATPase pump is necessary. Other enveloped RNA-containing viruses, such as vesicular stomatitis virus or influenza virus were also blocked by BFLA1, whereas no effect was observed with Sendai virus, which enters into cells by direct fusion with the plasma membrane. Enveloped DNA-containing viruses, such as herpes-viruses and vaccinia virus, infected the cells even when the vacuolar H(+)-ATPase was inhibited by BFLA1; similar behaviour was observed with poliovirus and adenovirus. Animal virus particles promote the internalization of proteins and other macromolecules during entry. BFLA1 blocked co-entry of the toxin alpha-sarcin when induced by SFV, but not when induced by Sendai virus. The inhibition of the enzyme responsible for acidification of endosomes by means of the potent inhibitor BFLA1 constitutes a selective and powerful tool to analyse the low-pH dependent mechanism(s) during virus entry and will aid in understanding the mechanisms and routes of entry of animal viruses into cells.