Forty-six patients with severe aplastic anemia (SAA), 1 with adult pure red cell aplasia (PRCA), 1 with congenital hypoplastic anemia (Diamond-Blackfan) and 1 with severe polymyositis were treated with intravenous antilymphocyte globulins (ALG) of different sources (2 of equine and 1 of rabbit origin). In all patients, direct and indirect antiglobulin tests (DAT and IAT) were performed, and in all patients treated with one type of equine ALG, positive DATs were found in Rh0(D)-positive patients, while the serum of Rh0(D)-negative patients treated with the same ALG reacted in vitro with Rh0(D)-positive erythrocytes. The antibody was eluted and shown to be of equine origin. Two patients suffered from frank heteroimmune hemolytic anemia. Since October 1984, the ALG of this particular source has not displayed any overt anti-erythrocyte activity any more. However, all clinicians treating patients with this type of immune immunosuppression should know that ALG may retain human erythrocyte (presumably anti-LW) activity.