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Patients were entered into a randomized trial of prophylaxis for renal allograft rejection by the administration of an anti—human IL-2 receptor antibody, anti-Tac, during the first ten days posttransplant. Interleukin-2 receptor (IL-2 R) expression was measured using two anti-IL-2 R monoclonal antibodies (moAbs), anti-Tac and 1HT4–4H3. These two antibodies recognize closely spaced epitopes on the 55 kD chain of the IL-2 R. IL-2 R expression was examined on peripheral blood small lymphocytes in three groups of patients who received: (A) cyclosporine CsA and prednisone for baseline immunosuppression (n=9); (B) anti-Tac with CsA and prednisone as baseline immunosuppression (n=12); and (C) anti-Tac with azathioprine and prednisone as baseline immunosuppression (n=5). We found that large numbers of T cells express IL-2 receptors despite the presence of anti-Tac (average of IL-2 R—positive cells at day of peak IL-2 R expression 56.0±20.8 in group A, 65.2±26.6 in group B, 21.0± 7.4 in group C). IL-2 R expression did not correlate with clinical activity, and the presence or accessibility of epitopes on the same 55 kD chain varied dramatically from patient to patient.