Normal human E rosette-forming, Fc-IgG receptor-bearing cells (so-called T gamma cells) were separated into two functionally different subpopulations. Both subpopulations bind the monoclonal antibody OKM1 (directed against an antigen present also on monocytes and granulocytes). The first subpopulation accounts for about 70% of the total T gamma cell population, does not bind OKT3 (a monoclonal antibody directed against an antigen present on most T lymphocytes), and displays strong killer (K) cell and natural killer (NK) cell activity. The second subpopulation accounts for about 30% of the total T gamma population, binds both OKT3 and OKM1 (confirmed with a double-labeling assay), and displays low K and NK cell activity. Each subset contained less than 10% null cells. Comparison of T gamma cell populations obtained by adherence to a monolayer of IgG-coated human erythrocytes or by rosette formation with these cells revealed that pure T gamma cells with normal killer cell and natural killer cell activity were best obtained with the monolayer technique. Comparison of the enzymic and functional profile of T gamma cells, monocytes and granulocytes, as well as changes during culture of these cells in vitro, failed to indicate a relationship between T gamma cells and cells of the myelomonocytic lineage.