The effect of weak bases (NH4Cl and amantadine) and carboxylic ionophores (monensin) on the infection of CD4 (T4) positive human cell lines by HIV-1 is examined. These reagents, which raise the pH of acidic intracellular organelles, fail to inhibit HIV-1 entry and the events leading to viral protein synthesis at concentrations inhibitory for low pH-dependent fusogenic enveloped viruses. The infectivity of VSV (HIV-1) pseudotypes is unaffected by weak bases at concentrations causing 95% plaque reduction of VSV in its own envelope. HIV-1 dependent cell--cell fusion (syncytium formation) occurs in medium maintained at pH 7.4-7.6, and virions are not irreversibly inactivated by incubation in acid medium. Our results show that HIV-1 entry and membrane fusion do not require exposure to low pH. The production of infectious HIV-1 particles, however, is inhibited in cells treated with NH4Cl.