Acquired amegakaryocytic thrombocytopenic purpura (AATP) is a disorder of hematopoiesis characterized by severe thrombocytopenia due to a selective reduction or total absence of megakaryocytes in an otherwise normal-appearing bone marrow. Although the development of autoantibodies directed against cells in the megakaryocyte progenitor cell pool has been implicated in the pathogenesis of this disorder, cell-mediated suppression of megakaryocytopoiesis has not been described. Accordingly, we report two cases of AATP in which in vitro suppression of megakaryocyte colony formation by autologous ancillary marrow cells was demonstrable. Light-density bone marrow mononuclear cells (MNCs) obtained from both patients were either plated directly into plasma clot cultures, or after first being depleted by adherent monocytes (M phi) or T lymphocytes using standard methodologies. In some experiments, the depleted ancillary marrow cells were recovered for autologous co-culture studies with the MNCs from which they had been depleted. Megakaryocyte colony formation was detected in the cultures using an indirect immunofluorescence assay with a rabbit anti- human platelet glycoprotein antiserum. Removal of M phi (n = 6), or T lymphocytes (n = 4) from normal marrow MNCs had no apparent effect on colony formation. In contrast, depleting T lymphocytes from the MNCs of patient 1 significantly augmented megakaryocyte colony formation; a similar effect was observed after depleting M phi from the MNCs of patient 2. This observed augmentation in colony formation could be abrogated by autologous co-culture with the putative suppressor cell at effector cell/target cell ratios of 1:10 in the case of T lymphocytes or 1:5 in the case of M phi. Neither suppression nor stimulation of megakaryocyte colony formation was observed after culturing normal MNCs with autologous T cells (n = 4) or M phi (n = 3) at similar or greater ratios. We also observed inhibition of megakaryocyte colony formation after culturing normal MNCs in the presence of tissue culture medium conditioned by the M phi of patient 2. This effect was shown to be specific for megakaryocytes since this same conditioned medium had no significant effect on BFU-E and CFU-E-derived colony formation by autologous marrow mononuclear cells. These results suggest that: both T cells and M phi are capable of exerting a regulatory effect on the proliferation of human megakaryocyte progenitor cells (CFU-Meg); in the case of M phi, a soluble factor elaborated by these cells may be responsible for suppressing CFU-Meg growth; and aberrant ancillary cell- megakaryocyte progenitor cell interactions may lead to clinically significant disease.