Monoclonal antibodies and a four-layer immunoperoxidase technique were used to analyze and quantitate the various infiltrating cell types found in percutaneous renal biopsies of 25 patients undergoing acute renal allograft rejection. Cell markers included monoclonal antibodies to the human leukocyte-common antigen (PHM 1), mononuclear phagocytes (PHM 2, FMC 32), T cells and T cell subsets (OKT 3, OKT 4, and OKT 8), B cells (7.2), polymorphs (FMC 10, FMC 13), and plasma cells (OKT 10). The proportion of labelled interstitial cells was expressed as a percentage of the total number of infiltrating leukocytes identified with PHM 1, and correlated with the histologically graded intensity of rejection. In mild rejection 32% of the infiltrating cells were T lymphocytes, of which 90% were OKT-8-positive cytotoxic-suppressor cells, and 52% were macro-phages. Similarly, in moderate rejection T cells composed 42% of the infiltrate (with 67% of T cells expressing OKT 8 antigen), and macrophages formed 38% of the total cells. By contrast, in severe rejection, the T cell component was decreased to 15% of the cells, of which 78% were OKT-8-positive; these were preponderantly macrophages (60%) and polymorphs (22%). These studies demonstrate that cytotoxic-suppressor T cells and macrophages are the major cells mediating acute interstitial graft rejection.