Chloroquine has been used as an anti-malarial drug and is known as a lysosomotropic amine as well. The effects of chloroquine on lysosomal integrity in cultured rat hepatocytes were studied by measuring lysosomal enzyme beta-glucuronidase (beta-G) or lysosomal membrane glycoprotein (lamp-1) in Percoll density gradient fractions, in the cytosolic fraction obtained from cells permeabilized by digitonin or in the cytosolic fraction obtained by conventional cell fractionation. The distribution of beta-G on a Percoll density gradient in chloroquine-treated cells was approximately similar to that of a cytosolic protein, mevalonate pyrophosphate decarboxylase, in nontreated cells. Lamp-1 was decreased in the lysosomal fractions on a Percoll density gradient in chloroquine-treated cells, and was increased in the plasma membrane fraction, as compared with the levels in nontreated cells. Furthermore, after cells were cultured in the presence and absence of chloroquine, the proportions of beta-G activity in the cytosolic fraction obtained from the digitonin-permeabilized cells were 19% and 4%, while those in the cytosolic fraction obtained by conventional cell fractionation were 54% and 26%, respectively. From these findings, we infer that chloroquine caused the disruption of lysosomes in the living cells, and that lysosomes treated with chloroquine were easily disrupted by homogenization or centrifugation during cell fractionation.