Among 98 patients with rheumatoid arthritis who were treated with the immunomodulating agent Levamisole 7 patients developed an agranulocytosis. The ethiology of the Levamisole induced agranulocytosis (LIA) is unknown however circulating granulocytotoxic antibodies are present in the sera of such patients. Routine examination of sera from Levamisole treated patients reveal that such antibodies can be detected several weeks before agranulocytosis in the peripheral blood occurs. Our observation suggest that circulating granulocytotoxic antibodies may play a significant role in the development of LIA and that determinations of such antibodies can provide an excellent tool for the early detection of high risk patients. Routine determinations of such antibodies will allow to increase the safety of this potent agent used in the treatment of rheumatic diseases and Tumors.