In this study, we investigated whether retinal soluble proteins, such as S-antigen, play a role in the induction of nitric oxide by macrophages in vitro. Cells from the murine macrophage cell line RAW 264.7 and rat and rabbit peritoneal macrophages were incubated in the presence of retinal soluble protein. The nitrite level in the cultured supernatant was measured to determine nitric oxide production using the Griess reaction. S-antigen induced significant, dose-dependent nitrite production in both RAW 264.7 and rat peritoneal macrophages. The induction of inducible nitric oxide synthase by retinal protein was inhibited by the iNOS-specific inhibitor, aminoguanidine and the tyrosine inhibitor, genistein. These results show that soluble retinal protein significantly induces nitric acid production by macrophages. Increased production of reactive oxygen species by macrophages in the presence of this soluble retinal protein in vivo may accelerate photoreceptor degeneration in uveitis.