Chloroquine improves drastically the transfection of cells upon exposure to plasmid DNA/glycosylated polylysine complexes. So far the mechanism of action of chloroquine is not well understood. In this paper, the effect of chloroquine was investigated by measuring the transfection efficiency of a human hepatocarcinoma (HepG2 cells) by pSV2LUC/lactosylated polylysine complexes involving their internalization via the galactose-specific membrane lectin of these cells. The luciferase activity in the transfected cells was maximal when the transfection was performed for 3 or 4 h in the presence of 100 microM chloroquine. The luciferase activity was also enhanced in the presence of primaquine, a chloroquine analogue, but was not increased when transfection was performed in the presence of ammonium chloride, methylamine, spermine, or monensin, compounds known to neutralize the pH of the endocytotic vesicle lumen as chloroquine does. Chloroquine enters cells and accumulates in vesicular compartments; the overall intracellular concentration increases to 9 mM, which means that in the vesicular compartment, the chloroquine concentration is still higher. At such high concentrations, chloroquine induces the dissociation of plasmid DNA/lactosylated polylysine complexes, as shown in acellular experiments.