A 66-year-old female was diagnosed to have severe aplastic anemia. Remission was not achieved by the ALG-oxymethorone therapy, and she was in need of RBC multitransfusion. After receiving a total of 42 units (16,800 ml) of red blood cells, it was found that her serum agglutinated strongly her own group BI cells at 4 degrees C. This agglutination disappeared in DTT solution. An eluate from her red cells also agglutinated BI panel cells, whereas Bi, O I, O i cells failed to react. It was apparent that the cold agglutinin in her serum had special affinity for I cells, which also contain B. Analysis of lymphocyte subset in peripheral blood showed T 3 (CD 3) 70.18%, T 4 (CD 4) 70.92%, T 8 (CD 8) 7.08%, T 4/T 8 = 10.1, T4 (+)2H4(+)/T4(+)2H4(-) = 1.05. Although relationship between appearance of anti-BI antibody and relative increment of CD 4 positive cells was not clear, we considered that these findings were caused by an abnormal autoimmune reaction in the patient with aplastic anemia.