A purified human monoclonal IgM(kappa) (cold agglutinin MKV) has been characterized as both a cold agglutinin and a cryoglobulin. Since its reactivity with human erythrocytes but not with dog erythrocytes is reduced by treatment of the cells with ficin and its reactivity with both is abolished by treatment of the cells with neuraminidase, it has by definition Pr(2) specificity [Roelcke, D. (1974) Clin. Immunol. Immunopath. 2, 266-280]. Presumably, the membrane receptors for cold agglutinin MKV are sialic acid-containing glycoproteins in human cells and sialic acid-containing glycolipids in dog cells. Agglutination of erythrocytes is specifically inhibited by II(3)-N-acetylneuraminosyllactosylceramide (GM(3)) and N-acetylneuraminosylparagloboside but not by their N-glycolylneuraminosyl forms or by lactosylceramide or paragloboside, and the reactivity of neuraminidase-treated cells can be restored by allowing them to absorb either GM(3) or N-acetylneuraminosylparagloboside. When large amounts of ganglioside are absorbed, the cells are agglutinated not only in the cold, but also at 37 degrees , showing that the density of receptor sites on the erythrocyte surface can influence the thermal amplitude of cold agglutinins. Liberation of sialic acid from cold agglutinin MKV by treatment with neuraminidase (acylneuraminosyl hydrolase; EC 3.2.1.18) does not affect its agglutinating properties but the asialoprotein is no longer a cryoglobulin. Apparently the physical basis for its precipitation in the cold is the intermolecular immune binding of N-acetylneuraminosyl residues.