Concanamycin A, a selective inhibitor of the vacuolar proton ATPase, blocks the infection of animal cells by vesicular stomatitis virus, Semliki Forest virus and influenza virus even when the drug is present at the low concentration of 5 nM. Nevertheless the antibiotic prevents neither the attachment, to cells, of Semliki Forest virus nor its subsequent internalization. Under certain conditions, described in this communication, virus entry is prevented even when the pH of the medium is low, thus suggesting that a pH gradient, rather than low pH per se, is required to drive the entry, into cells, of these enveloped animal viruses.