AbstractFrom a patient, a new type of human cold agglutinin, Lud, which reacts with a determinant involving N‐acetylneuraminic acid and which is not fully expressed at birth, is described. This antibody differs from a recently described cold agglutinin, Fl. The antigenic determinant recognized by antibody Lud is diminished after treatment of erythrocytes with proteases, whereas that recognized by Fl is no