The immunopathologic response of guinea pigs to sensitization with retinal ‘s’ antigen has a biphasic character that is dose-dependent. At low dose levels, a non-necrotizing posterior segment inflammation is observed. At high dose levels an acute necrotizing panophthalmitis is observed. Complement depletion markedly modulates the inflammation observed following sensitization with high dose levels but appears to have no effect on the outcome of the low dose sensitization. The dose-response correlation in the inflammatory reactions induced by sensitization with retinal ‘s’ antigen may reflect the role of antibody when high doses of ‘s’ antigen are employed and the role of cell mediated immunity where low doses of antigen are employed.