The effect of triidothyronine (T3) on the responses to mitogens and on the production of prostaglandin E2 and interleukin 2 were studied in serum-free cultures of peripheral blood mononuclear cells (PBMC) in 20 patients undergoing hemodialysis and in 30 control subjects. T3 increased the growth of PMBC induced by phytohemagglutinin and pokeweed mitogen in both groups. PBMC reached growth maximum at 0.5 nM T3 when stimulated by phytohemagglutinin in both groups. At higher concentrations of T3 the effect declined in the control group, but the response of uremic PBMC was constant. The response to T3 of pokeweed mitogen stimulated PBMC was lower in the uremic patients. The production of prostaglandin E2 by PBMC was higher in the uremic patients than in the controls. T3 had no effect on prostaglandin E2 production. Indomethacin alone and in combination with T3 had a stimulatory effect on cell growth in the patient group. T3 had no effect on the release of interleukin 2 by PBMC. An additive effect of interleukin 2 and. T3 was observed in cultures stimulated by suboptimal concentrations of the mitogens. In conclusion, the impaired growth of PBMC in serum-free cultures from uremic patients was enhanced, however, not normalized, by external addition of T3, inhibition of prostaglandin E2 synthesis, and addition of interleukin 2.