Fifty-five renal transplant recipients were studied prospectively for changes in monoclonal antibody-defined mononuclear cell subsets (MCS) over the first 45 days posttransplant. Patients received induction immunotherapy with either monoclonal OKT2(n = 29) or polyclonal RATS (n = 26) preparation. Sequential examinations showed characteristic patterns of MCS depletion, which differed according to the type of therapy received and which subset was examined. In general, induction therapy with RATS resulted in greater and more sustained reduction of most MCS than was seen with OKT3 therapy. In recipients who received OKT3 induction there was a correlation between allograft rejection and an increase in lymphocytes expressing pan-T cell markers and in natural-killer cells. In contrast, rejection episodes in patients receiving RATS were associated with increases in subpopulations of T cells including helper, inducer and suppressor/cytotoxic T-cell subsets. There was no correlation of rejection with B cells or T-cell activation markers. The different patterns of MCS depletion with different antilymphocyte preparations and the association between changes in different MCS and rejection warrant further investigation.