We studied a patient with a rare complication of amegakaryocytic thrombocytopenia (AMT) associated with systemic lupus erythematosus (SLE). To investigate the underlying pathogenesis of AMT, the effects of peripheral blood T cells and serum on human megakaryocyte progenitor cells were studied using in vitro coculture techniques. Mononuclear bone marrow cells (2 X 10(5) from normal donors produced 33.6 +/- 8.8 (n = 10) colony-forming unit-megakaryocytes (CFU-M) in our plasma clot system. When 2 X 10(5) of the patient's T cells were added to the culture system, the number of CFU-M decreased to only 3.5 +/- 0.6/2 X 10(5) bone marrow cells. No evidence of inhibitory effects was found by the addition of the patient's serum and complement to the culture system. The T cells stored at -80 degrees C on admission were also capable of suppressing autologous CFU-M after recovery from AMT. These results indicate that in vitro suppression of CFU-M from allogenic and autologous bone marrow cells by this patient's T cells provides an explanation for the pathogenesis of AMT associated with SLE.