A human homogeneous IgM/K cold agglutinin (CA) Sa is described, whose corresponding antigen on erythrocytes (RBC) was abolished by neuraminidase. This indicated that the antigen was related to N- acetylneuraminic acid, similar to Pr and Gd antigens. In contrast, this antigen was only partially destroyed by proteases, whereas Pr antigens are completely destroyed and Gd antigens are not influenced by proteases. Sa antibody activity was inhibited by sialyllactose NeuAc (alpha 2 leads to 3) (alpha 2 leads to 6) Gal (beta, 1 leads to 4) Glc like anti-Gd but in contrast to anti-Pr. The corresponding antigen was associated with an RBC membrane glycoprotein fraction like Pr, Sa is one of a spectrum of human monoclonal CA against cell surface neuraminyl groups.