A wide range of cell populations were examined for Fc receptor (FcR)-bearing T cells: thymus, spleen, peritoneal cells, and T cells activated to H-2 antigens in spleen (ATC spleen) and in thoracic duct lymph (T-TDL). In addition, B lymphocytes from thoracic duct lymph of athymic nude mice and a Thy-1-positive, FcR-positive thymoma served as control cell populations. Reagents used were aggregates of human gamma-globulin and of various mouse myeloma proteins (IgG1, IgG2a, IgG2b), radioiodinated antigen-antibody complexes, and sheep erythrocyte antibody rosettes. Labeling techniques involving radioautography and immunofluorescence were used to demonstrate FcR by one of the above reagents and to identify T cells either by staining with anti-Thy-1.2 or by a specific rabbit anti-mouse T cell serum, or by failure to stain with anti-mouse immunoglobulin. In some experiments phagocytic cells were removed whereas in others they were identified by their capacity to engulf latex particles. Approximately 25% of cells with T cell markers were FcR-bearing cells in thymus, normal spleen, and peritoneal cavity, and 17% in ATC spleen. FcR on T cells in peripheral lymphoid tissues were detectable by aggregates of HGG and myeloma proteins and by radioiodinated immune complexes. Those on T cells in thymus were revealed only by aggregates of HGG. Circulating T cells (T.TDL) failed to display FcR: a) despite the use of a wide range of the above labeling techniques, each of which was shown to detect FcR on other T cells, thymoma cells, and B cells, and b) even after removal of Ig associated with their cell membranes. In contrast to B cell FcR which bound IgG1 preferentially, those on T cells bound both IgG1 and IgG2, raising the possibility that the FcR on T cell is distinct from that on B cell. It is concluded that FcR-bearing T cells represent a subpopulation of cells within the thymus and the secondary lymphoid tissues.