It has been shown in the mouse that antigen triggers distinct subpopulations of thymusderived lymphocytes to express specific immunoregulatory activities, and that these activities are mediated in part by intercellular signals involving products of the I-region (Ia antigens) of the major histocompatibility complex. Thus, discrete Ia antigens have been detected on soluble T-cell factors which have either helper (1) or suppressor (2) activities, and are also present on the surface membrane of suppressor T cells (3), and Con A-induced T-cell blasts (4).Although Ia antigens have been generally thought not to be expressed by normal or leukemic T cells in man (5, 6), this view was re-examined when we found that antibody to a human Ialike antigen, p23,30 occasionally reacted with leukemic blasts which also expressed thymusdependent markers, p23,30 is a glycoprotein complex of 23,000 and 30,000 dalton subunits, isolated from the papain-solubilized membrane of a human lymphoblastoid B-cell line (7). A rabbit anti-p23,30 serum binds to peripheral B cells, monocytes, and a subpopulation of null cells, but is unreactive with normal human T cells or thymocytes (6,8). In addition to conforming in molecular weight and normal tissue distribution with murine Ia antigen, the detergent-solubilized form of p23,30, p29,34, reacts with B-cell alloantisera which are specific for determinants coded by the HLA-D locus, or I-region counterpart in man (9).We report here that determinants recognized by anti-p23,30 are expressed on the surface membrane of T cells which are transformed by alloantigen in mixed leukocyte culture (MLC), but are not detectable on T cells which are either freshly purified or maintained without stimulation in culture for 6 days. Moreover, allosensitized T cells were shown to elaborate and to incorporate into their surface membranes a 29,000 and 34,000 dalton, HLA-D-related complex, Materials and MethodsIsolation of Human TLymphocytes. T cells were isolated from peripheral blood mononuclear cells by nylon wool purification followed by formation and density gradient sedimentation of sheep erythrocyte-rosetting cells (10).Sensitization Cultures. T cells were sensitized to aUogeneic mitomycin-C-treated mononuclear cells (10). Of the cells recovered after a 6-day sensitization, greater than 98% were reactive with a heterologous antiserum which was specific for thymus-derived lymphocytes.

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