Some cold autoantibodies found in acquired hemolytic anemias react with both the A, B or H, and the I, blood group antigens. The agglutinating activity of such an autoantibody of the IgM type was studied by a quantitative method in an AB patient. This antibody agglutinated neither A, B and O red blood cells in the absence of the 1 antigen, nor the Bombay red cells. It agglutinated equally AI + and BI + red cells, and weakly the OI+ red cells. Both agglutinating activities towards the AI+ and BI+ red cells were observed to decrease simultaneously and partially upon absorption on OI + red cells, at 4°C. They disappeared simultaneously and completely upon absorption on either AI + or BI + red cells. The antibody absorbed on, and eluted from AI + red cells showed a similar agglutinating activity towards the AI + and the BI + red cells ; likewise, when it was absorbed on, and eluted from BI + red cells. The study of the antibody-agglutinating activity and of its 7S subunit activity, as well as of the inhibitory activity of the Fab fragments, led to the conclusion that the observed results were due neither to a mixture of antibodies, nor to an anti-I or an anti-HI antibody, but to a cross-reactivity between the A, B and I antigens. The antibody recognized a determinant which comprises I and some part shared by A and B. It can be named anti-I (A + B).