Two patients with disseminated mycobacterial infection presented with severe neutropenia and hematopoietic failure. Marrow cells were obtained from each patient and were cultured in methylcellulose before and after the removal of mononuclear phagocytes, T-lymphocytes, or both from the marrow cell suspension. Glucocorticosteroid-resistant T-lymphocytes markedly inhibited granulopoiesis, but mononuclear phagocytes did not. Indomethacin inhibition of prostaglandin synthesis did not influence suppression of colony growth by the inhibitory lymphocytes. In the one patient who responded favorably to antituberculous therapy, the in-vitro T-lymphocyte inhibition of granulopoiesis disappeared as the neutropenia resolved. Thus, in some patients with disseminated mycobacterial infection, clinical bone marrow failure may be mediated, at least partly, by T-lymphocytes that suppress hematopoiesis.