A girl with a history of autoimmune disease developed life-threatening agranulocytosis. A bone marrow biopsy demonstrated selective granulocytic hypoplasia. No antineutrophil antibodies were found. In vitro bone marrow culture of granulocytic progenitor cells suggested T cell-mediated inhibition of colony formation, which was reduced by in vitro treatment of marrow cells with either hydrocortisone or an antibody directed against T-lymphocytes and complement. The patient responded to treatment with antithymocyte globulin after administration of corticosteroids and other immunosuppressants failed to increase her neutrophil count significantly. Attempts to stop ATG treatment resulted in precipitous drops in her neutrophil counts, which reversed with readministration of ATG. She then received weekly ATG infusions for over 24 months until she was able to maintain a normal neutrophil count. A trial of ATG therapy may be indicated in severe neutropenia when in vitro culture results indicate a possible autoimmune basis.