The functional relationship in vitro between mitogen-induced lymphocyte transformation, lymphocyte response to interleukin-2 (IL-2) and steroid, and production of IL-2 was examined in patients with chronic renal failure on hemodialysis (HD) or on continuous ambulatory peritoneal dialysis (CAPD). The lymphocyte responses to optimal stimulation with phytohemagglutinin, concanavalin A, and pokeweed mitogen were depressed in lymphocyte cultures from HD patients, while CAPD lymphocyte cultures responded normally. However, at suboptimal phytohemagglutinin stimulation both CAPD lymphocyte and HD lymphocyte responses were subnormal. Uremic lymphocyte cultures were more sensitive to the immunosuppressive effect of methylprednisolone. Addition of IL-2 normalized the phytohemagglutinin responses of suboptimally stimulated CAPD lymphocyte cultures and clearly improved the mitogen responses of the HD lymphocyte cultures. Furthermore, the increased uremic lymphocyte sensitivity to methylprednisolone was normalized by addition of IL-2 to the cultures. The measured IL-2 production had clearly decreased in the HD cultures after 48 h as compared to that of the control cultures. A similar but not significant trend was also seen in the CAPD cultures. Thus, it is suggested that a deficient production of IL-2 may partly explain the reduced lymphocyte response of uremic lymphocytes in vitro.