Antithymocyte globulin (ATG) is frequently effective therapy for aplastic anemia. Its mechanism of action is often assumed to be upon a lymphocyte inhibitor of hematopoiesis. However, specificity for T lymphocytes would not be anticipated from consideration of the method of preparing ATG. In fact, using flow microfluorometry and fluorescence immunohistochemistry, we have found that ATG binds to virtually all circulating lymphocytes, granulocytes, and platelets, as well as to bone marrow cells. Extensive absorption of ATG with either granulocytes or lymphocytes does not eliminate reactivity with the opposite cells, indicating that ATG recognizes some distinct antigens on each cell type. Treatment of cells with ATG blocks the binding of monoclonal antibodies directed against either lymphocyte differentiation or histocompatibility antigens. ATG also binds to visceral tissues, including thymus and testis cell membranes and the nuclear and cytoplasmic components of tonsil, kidney, liver, breast, lung, and intestine. In vitro cytotoxicity of ATG was demonstrated for both T and non- T lymphocytes and platelets. Despite its name, ATG is not specific for a particular cell type, and it would be premature to conclude that its effect is mediated through a specific lymphocyte population.