Abstract Certain hemolytic disorders are characterized by the attachment of a globulin to the erythrocyte membrane, this globulin possessing the characteristics of an antibody for antigenic determinants associated with the surface of the red cells (1). Cells with attached globulin, i.e., sensitized cells, may agglutinate spontaneously when placed in serum, albumin solutions, or other protein-containing mediums, but usually do not agglutinate when suspended in saline. Erythrocytes, so sensitized, are encountered in the blood of some patients with idiopathic acquired hemolytic anemia and erythroblastosis fetalis (2–4). Similar sensitization may follow exposure of normal red cells in vitro to nonagglutinating antibody, occasionally present in the serum of those “iso-immunized” by pregnancy or transfusion (5–7). The adherent globulin may be demonstrated most readily by the so-called “Coombs test,” the basis of which is the agglutination of these cells by an antiserum prepared in rabbits injected with human serum.