The effect of experimentally induced uremia in the rat on the synergistic response between thymus cells (TC) and lymph node cells (LNC) was examined. It was found that: (1) Uremic TC and LNC interact in a synergistic fashion which is greater than that observed for control cells; (2) The response of uremic LNC to alloantigens is suppressed when compared to the response of control LNC; (3) Uremic TC provide more help to control LNC in their response to alloantigens than do control TC; and (4) Treatment of uremic rats with cortisone acetate (CA) enhances their TC ability to amplify control LNC response to alloantigens. Thus, it appears that, while the response of uremic LNC to alloantigens is markedly suppressed, there are potent amplifier cells present in the thymus of uremic rats which have the ability to act in synergism with control LNC in response to alloantigens. This effect is significantly greater than the synergistic activity of control thymocytes.