Cellular infiltrates in 20 renal allograft biopsies taken at the time of acute rejection episodes were analyzed with antibodies reactive with monocytes (BRL antihuman monocyte monoclonal antibody), T lymphocyte subpopulations (OKT- and Leu-series of monoclonal antibodies), and B lymphocytes (heterologous antihuman IgM antibodies). For demonstration of the various mononuclear cells, an indirect immunoperoxidase technique was used. The number of monocytes in the infiltrates varied from 10–20%; the number of B lymphocytes was always below 10%. The T lymphocytes accounted for 50–90% of the total number of mononuclear cells. The OKT4/OKT8 ratios for the T lymphocytes in the graft infiltrates were correlated with the peripheral blood OKT4/OKT8 ratios measured by indirect fluorescence and flow cytometry. The OKT4/OKT8 ratios for perivascular infiltrates correlated far better with peripheral blood OKT4/OKT8 ratios (r=0.72) than did the OKT4/OKT8 ratios for interstitial infiltrates (r=0.58). Low or inverted OKT4/OKT8 ratios and low or inverted Leu 3a/Leu 2a ratios were associated with a high risk of irreversible graft rejection (P