The effect of blood transfusions, given in low number (less than 5), on the immune response of renal dialysis patients was studied. A significantly lowered response of lymphocytes to mitogen stimulation was observed in patients after as few as one, two or three transfusions, depending on the patient. This led to an increased delta 48/delta 0 ratio reflected by the enhanced response of the cells following delayed addition of suboptimal dose of mitogen. There was no modification of the ratio of helper/inducer to suppressor/cytotoxic T cells subsets (OKT4/OKT8). The use of such simple in vitro tests in a strict protocol of transfusions could allow an adequate follow-up thereby limiting the risks of sensitization. These results demonstrate that important phenomena affecting patients' immune response are turned on following even a low number of transfusions in the majority of uremic patients. This could probably be related to the beneficial effects of blood transfusions on the kidney allograft survival already described.