The effect of five lysosomotropic weak bases (chloroquine, amantadine, tributylamine, methylamine and NH4C1) on Semliki Forest virus (SFV) infection has been studied in BHK-21 cells. When present at concentrations equal to or greater than 0.1, 0.5, 2, 15 and 15 mM respectively, the agents inhibited SFV infection by more than 90%. The effect was reversible and involved a process occurring within the first 60 min of virus-cell contact. The agents did not have a direct virucidal effect nor did they affect virus binding to the cells, receptor-mediated endocytosis of prebound virus, intracellular distribution of virus after endocytosis, or the low pH-induced membrane fusion activity of the virus spike glycoproteins. The step blocked by chloroquine and NH4C1 occurred intracellularly and was identified as the release of the virus nucleocapsid into the cytoplasm or the uncoating process. On the basis of these results, our previous studies on SFV entry, and the known effects of lipophilic amines on lysosomes, we conclude that the agents affect entry by a common mechanism: they prevent the transfer of the virus nucleocapsid into the cytoplasm by increasing the lysosomal pH above the critical value needed to trigger a low pH-dependent fusion reaction between the membranes of the lysosome and the virus.