Spleen cells from chimeric NOD mice have suppressive activity against autoreactive cells. Naive NOD responder spleen cells (0.1 × 10⁶) were harvested and plated with 0.1 × 10⁶ irradiated NOD stimulator spleen cells. Stimulation index was then normalized to the proliferation detected by [³H]-thymidine incorporated in these cultures. To evaluate the effect of spleen cells from the various conditioned NOD BMT mice, spleen cells were harvested and added to the responder/stimulator cells either 0.1 × 10⁶ spleen cells (1:1 ratio) or 0.05 × 10⁶ spleen cells (1:0.5 ratio). The anti-CD8/600 TBI-treated mice and the no-mAb/750 TBI mice were all chimeric. None of the anti-CD8/αCD4/600 TBI treated mice were chimeric (nonchimeric), and an increase in stimulation is noted when these cells were added to the cultures. Spleen cells obtained from these NOD mice that received transplants were combined with the responder NOD spleen cells at either a 1:1 or a 1:0.5 ratio. Data represent the means obtained from 4 individual spleen cell preparations per BMT group obtained over 2 separate experiments. Horizontal line illustrates the controls on which the stimulation indexes were calculated. Cultures with means above the line proliferated more than the controls, whereas cultures with means below the line proliferated less than the controls.