Using human peripheral lymphocytes activated by concanavalin A (ConA), we observed suppression of the normal lymphocyte response to mitogens and allogeneic cells. The suppressive effect requires at least a 24 hr activation period of lymphocytes with ConA and is dose dependent. Activation of the suppressor population and stimulation of the responding population with mitogenic doses of ConA caused suppression of the response to mitogens, whereas activation and stimulation with submitogenic doses generally caused enhancement of the response. Responding lymphocytes showed individual variation in their ability to be suppressed by ConA-activated lymphocytes. This variable suppression could be demonstrated in autologous or allogeneic lymphocytes stimulated by ConA and also in the mixed lymphocyte reaction. ConA- activated lymphocytes derived from an aging human population differ from those of young individuals in showing a variable loss of suppressor activity. The decline of this suppressive phenomenon in aged humans may provide an explanation for the increased frequency of autoantibodies seen in the aging human population.

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