The possible antiproliferative effect of melatonin on retinal pigment epithelial (RPE) cells in vitro was investigated. Bovine RPE cells cultured in Ham's F12 medium supplemented with 10% fetal bovine serum had a nuclear density of 73.6 +/- 6.1 nuclei/mm2 at 72 h after seeding. The nuclear density at this time-point was doubled if either 50 or 100 ng/ml human epidermal growth factors (hEGF) was added to the culture medium. When these hEGF-stimulated cells were treated with melatonin from 10 to 500 pg/ml, the proliferation was suppressed with a dose-response relationship. At 250 and 500 pg/ml melatonin, the nuclear densities of the melatonin-treated cells were similar to those of the control cells. Using mitotically active SV-40 transformed human fetal RPE cells cultured in a serum-free medium, melatonin was also shown to be antiproliferative. In the presence of 500 pg/ml melatonin, the proliferation of these cells was inhibited to 77% as compared to the control. These results were further supported by the reduced [H3]thymidine uptake in the melatonin-treated cells. We propose that melatonin, at physiologic concentrations, has an antiproliferative effect, and that cultured RPE cells stimulated to proliferate by either hEGF treatment or SV-40 transfection are responsive to melatonin. Melatonin may either inhibit mitosis in actively dividing cells or modulate hEGF action.