The production of gamma-interferon (IFN gamma) in human peripheral blood T lymphocytes was induced by stimulation with PHA VSports手机版. For identification of the producer cell of IFN gamma, double fluorescence studies were undertaken and titers of interferon were determined in preparatively separated T-cell subpopulations reactive with one of the monoclonal antibodies OKT3, OKT4, OKT8, and OKIal . Production of IFN gamma was found in OKT3+, OKT4+, and OKT8+ cells. However, IFN gamma production occurred only in T cells also reactive with the monoclonal antibody OKIal . Addition of macrophages had no substantial effect on interferon titers in these subpopulations. It is suggested that the T cell subset producing IFN gamma is characterized by its reactivity with the monoclonal antibodies OKT3, OKT4 or OKT8, and OKIal . .