Immunological studies were performed with the expanded T gamma cells of five patients with T gamma lymphocytosis. All patients showed a stable clinical picture with persistent T gamma lymphocytosis and neutropenia, with or without recurrent infections. The expanded T gamma cells in the blood had the T1-3+4-8+11+M1- phenotype, with the exception of one patient whose cells lacked the T8 marker. The expanded T gamma cell population did not proliferate in response to T cell mitogens and did not show immunoregulatory activity on pokeweed mitogen driven immunoglobulin synthesis. In four of the five patients the T gamma cells had killer cell activity against IgG sensitized mouse mastocytoma cells. Taken together, these results and the data from the literature, it is concluded that T gamma lymphocytosis represents a spectrum of T cell expansions clearly distinct from clinically progressive mature T cell neoplasias.