We investigated the effect of free radical scavengers on the actions of cytokines on islet cells. Interferon-γ and tumor necrosis factor-α reduced the nicotinamide adenine dinucleotide content of mouse islet cells; the combination of interferon-γ (4×105 U/I) and tumor necrosis factor-α (4×105 U/I) caused nicotinamide adenine dinucleotide reduction by ∼40%. Dimethyl urea and dimethyl sulfoxide prevented the decrease, whereas superoxide dismutase, catalase, and mannitol were not effective. Dimethyl urea and dimethyl sulfoxide protected islet cells from the synergistic cytotoxic action of interferon-γ and tumor necrosis factor-α. Major histocompatibility complex class II antigen induction by interferon-γ and tumor necrosis factor-α was also inhibited by dimethyl urea and dimethyl sulfoxide, but not by superoxide dismutase, catalase and mannitol. Since superoxide dismutase of a membrane-penetrable form attenuated the class II antigen induction, the inefficiency of superoxide dismutase, catalase and mannitol may be attributable to their inability to penetrate islet cells. These results suggest that the intracellular generation of free oxygen radicals is involved in islet cell cytotoxicity and class II molecule expression by interferon-γ and tumor necrosis factor-α, and that nicotinamide adenine dinucleotide reduction may be associated with islet cell dysfunction caused by the cytokines.