To determine the effects of activated monocytes on the induction of human retinal pigment epithelial (HRPE) cell reactive oxygen metabolite (ROM) production and apoptosis.HRPE cells were co-cultured with interferon-gamma (IFN-gamma)-stimulated human monocytes. HRPE apoptosis was detected by propidium iodide, proliferating cell nuclear antigen (PCNA) and TdT-mediated dUTP nick end labeling (TUNEL) staining, caspase-3 activation, and Western blot analysis. HRPE cell ROMs were imaged using the fluorescent marker dihydrotetramethylrosamine (H2TMRos).IFN-gamma-activated monocytes in direct contact with HRPE cells elicited significant increases in TUNEL-positive (P < .0001) and decreases in PCNA-positive (P < .0001) HRPE cells. The activated monocytes also induced HRPE cell caspase-3 activation, which was inhibited by inhibitor Z-DEVD-fmk. Co-incubations, in which monocytes were either prevented from direct contact with HRPE cells or separated from HRPE cells after 30 minutes of direct contact, did not induce significant HRPE cell apoptosis. Anti-CD18 and anti-ICAM-1 antibodies significantly reduced activated monocyte-induced TUNEL-positive HRPE cells, by 48% (P = .0051) and 38% (P = .046), respectively, and caspase-3 activity by 56% (P < .0001) and 45% (P < .0001), respectively. Overlay of monocytes induced HRPE cell ROM that was inhibited by anti-CD18 and anti-ICAM-1 antibodies, but not by superoxide dismutase (SOD) or nitric oxide (NO) inhibitors. Accordingly, neither SOD nor NO inhibitors had significant effects on HRPE cell apoptosis or caspase-3 activation.We demonstrated that IFN-gamma-activated monocytes may induce ROM in HRPE cells through cell-to-cell contact, in part via CD18 and ICAM-1, and promote HRPE cell apoptosis via caspase-3 activation. These mechanisms may compromise HRPE cell function and survival in retinal diseases in which mononuclear phagocyte infiltration at the HRPE interface is observed.