Abstract: Retinal pigment epithelial cells carry out phagocytosis and digestion of material shed from the photoreceptor outer segments. In this process, the integrity of lysosomal enzymes is of major importance. In the present study the effects of tamoxifen, toremifene and chloroquine on the activity of two lysosomal enzymes (cathepsin D and N-acetyl-β-D-glucosaminidase) in the retinal pigment epithelial cells were studied. Retinal pigment epithelial cells from pig eyes were cultured for two weeks in Dulbecco's Modified Eagle Medium, after which the cells were exposed to 1–40 μM concentrations of tamoxifen citrate, toremifene citrate and chloroquine diphosphate. To eliminate possible medium-borne oestro-genic mechanisms, the test was repeated using phenol red-free medium with charcoal-stripped fetal calf serum. The exposure time was one week, after which the lysosomal enzymes cathepsin D and N-acetyl-β-glucosaminidase were determined. Cellular injuries were assessed by quantifying the leakage of lactate dehydrogenase into the culture medium. Cathepsin D and N-acetyl-β-D-glucosaminidase showed different sensitivities to tamoxifen, toremifene and chloroquine. The main lysosomal protease cathepsin D was more sensitive than N-acetyl-β-D-glucosaminidase to the effects of tamoxifen and toremifene, possibly due to their antioestrogenic properties. The phenol red-free medium with charcoal-stripped serum seemed to make the drugs more effective than the reference medium. Chloroquine had only a minor effect on the lysosomal protease cathepsin D, but a clearer effect could be seen on N-acetyl-β-glucosaminidase.