We have quantitated the impact of post-transplantation uremia on the antiallograft immune response by transplant aspiration cytology. Sixty-four consecutive renal transplants, treated with a similar immunosuppresive regimen, were aspiration biopsied at 2-day intervals during the first 15 days postoperatively. The patients were allocated into three groups on the basis of their serum creatinine level on the 3rd postoperative day: transplants with a delayed onset of function (highly uremic group, serum creatinine level greater than or equal to 600 mumol/liter; 24 cases), transplants with a partially delayed onset of function (partially uremic group, 200 to 600 mumol/liter; 21 cases), and transplants with an immediate onset of function (nonuremic group, less than or equal to 200 mumol/liter; 14 cases). These three groups were comparable in respect to the mean age, sex ratio, number of HLA-ABC mismatches (DR was not typed), number of pretransplant blood transfusions, and underlying diseases. Seventy percent of the transplants in the high uremic group, 60% in the moderately uremic group, and 60% in the nonuremic group underwent an early inflammatory episode during days 0 to 15 post-transplantation. The date of onset of inflammation was not significantly different in the three groups. However, the size and type of inflammation were significantly different: compared with the transplants in nonuremic patients, the total inflammatory response was slightly (P = 0.272) depressed in the transplants of moderately uremic patients and significantly (P = 0.007) depressed in the transplants of highly uremic patients. This depression was attributable to the depression of the blastogenic response: compared with nonuremic patients the blastogenic response was distinctly (P = 0.059) depressed in the moderately uremic group and significantly (P = 0.003) depressed in the highly uremic group. Instead, the frequency of in situ macrophages was the same in the three groups, or moderately elevated in the highly uremic group (P = 0.079). However, the graft survival was only 40% in the highly uremic group compared with 79% in the nonuremic controls and 81% in the moderately uremic patients (P = 0.016). We conclude that post-transplantation uremia partially impairs the antiallograft immune response, but this impairment is so small that other factors, whose nature cannot be explained on the basis of the present results, overrule the effects of uremia on graft survival.