The in vitro immune response of lymphocytes from uraemic patients was studied by comparing the in vitro kinetics of interleukin 2 (IL-2) production, the mitogen-induced proliferative response, and the expression of IL-2 receptors by T lymphocytes. The IL-2 production in 26 uraemic cell cultures decreased significantly after 48 h of stimulation with mitogen compared with that of 24 control cultures. The lymphocyte responses to phytohaemagglutinin (PHA), concanavalin A (Con A), and pokeweed mitogen (PWM) increased linearly with time, but the responses of the uraemic cell cultures were significantly lower than those of the control cultures. The relative numbers of cells double-stained for both Tac (IL-2 receptor)/HLA-DR or Tac/Leu 2 were significantly increased in the uraemic cultures as compared with the control cultures at 48 and 72 h. A similar, but not significant, trend was also demonstrated for uraemic cells positive for Tac/Leu 3. These findings were also seen in uraemic lymphocyte cultures supplemented with exogenous IL-2. Thus, the IL-2 production of uraemic lymphocytes seems to be exhausted more rapidly than that of normal lymphocytes, and there is no evidence that the poor proliferative response of uraemic lymphocytes is due to a decreased relative number of cells positive for IL-2 receptors.