Summary.A new cold reactive antibody directed against fetal (i) cells is described in 90% of patients with infectious mononucleosis. It appears to be the 7S counterpart of the previously described 19S anti‐i antibody. The 7S anti‐i antibody interacts with a 19S anti γ globulin and the combination is responsible for the majority of the cold agglutinin phenomena previously shown in this disorder. The highest titers were detected in patients with severe hemolytic anemia, leukopenia, thrombocytopenia or combinations the