A monoclonal anti-Tac antibody has been identified as a putative antibody against the human interleukin 2 (IL 2) receptor. In the present study, anti-Tac antibody was used to determine the location of cells expressing IL 2 receptors in frozen sections of human lymph nodes and tonsils by means of an immunoperoxidase technique. It was found that a substantial number of lymphoid cells reactive with anti-Tac antibody were present in these tissues. The majority of the Tac-positive cells were located in the paracortical and interfollicular regions of lymph nodes and tonsils, whereas only a few Tac-positive cells were scattered in the mantle zones and germinal centers of the secondary follicles. In contrast, no Tac-positive cells were demonstrated on cytocentrifuge preparations of peripheral blood lymphocytes from some of tissue donors, as evaluated by the same technique. In some experiments, a double-marker immunofluorescence analysis with the use of different fluorochromes, fluorescein isothiocyanate (FITC), and tetramethylrhodamine isothiocyanate (TRITC) was applied to characterize the phenotypes of cells expressing Tac antigen. Double staining with TRITC and FITC, respectively, for the identification of Tac-positive cells and T cells, showed that Tac-positive cells in lymph nodes and tonsils almost exclusively co-expressed a pan-T cell marker, Leu-1 antigen, that probably does not belong to non-T cell lineages. About 80% of Tac-positive cells were Leu-3 (helper/inducer) positive, and 20% of them Leu-2 (suppressor/cytotoxic) positive. These observations imply the plausible notion that an IL 2-mediated immune activation of T cells may actually occur in local lymphoid organs.