We have recently described several monoclonal antibodies (mAb) that recognize a heterodimeric structure (gp33/27,000 MW) expressed on the surface of human peripheral blood T lymphocytes upon activation with different mitogenic stimuli. Such mAb, when used in combination with submitogenic doses of phorbol ester, were capable of triggering T-cell proliferation. The antigen has been designated as activation inducer molecule (AIM). In the present study we have investigated the expression of the AIM in different lymphoid and non-lymphoid tissues. In addition, we have analysed the ability of lymphocyte subsets derived from thymus and tonsil to proliferate in response to anti-AIM mAb. The presence of AIM on subpopulations of lymphoid cells from thymus, tonsil, lymph node and spleen has been demonstrated by immunoprecipitation, flow cytometry and immunoperoxidase staining of tissue sections. By contrast, non-lymphoid cells from tissue such as brain, kidney, liver, lung or skin did not react with anti-AIM mAb. In thymus, the AIM expression was restricted to a subset of CD3+ medullary thymocytes, whereas CD1+ CD3-cortical thymocytes did not express this antigen. Nevertheless, the majority of both purified CD1-and CD3-thymocytes expressed AIM antigen after treatment with PMA. In tonsil and lymph node, a strong staining of a subset of CD3+ T lymphocytes located in the germinal centre was observed by immunohistochemical labelling with anti-AIM mAb. Certain T cells from the paracortical zone and CD19+ B lymphocytes from mantle region were also reactive. Both purified tonsillar T and B lymphocytes strongly expressed AIM after activation with PMA. The anti-AIM mAb was able to induce a strong proliferative response on purified CD1-thymocytes as well as on both purified tonsillar T and B lymphocytes in the presence of submitogenic doses of PMA. By contrast, no proliferative response was induced through the AIM in the CD3-immature thymocyte subset.