The mechanism of cryoprecipitation of a monoclonal IgM kappa cryoglobulin (Mou) with a cold agglutinin activity of Pr2 specificity has been studied. By immunodiffusion this cryoglobulin reacted (by its Fab' fragment) with micellar GM3, a ganglioside bearing the Pr2 antigenic determinant. In contrast to previous reports that indicated a possible temperature dependent self-association of IgM molecules via an immunological interaction leading to cold precipitation, we could not detect any affinity of this cryoglobulin for IgM when we used passive hemagglutination or an indirect enzyme-linked immunosorbent assay (ELISA). However, a GM3-like ganglioside could be extracted, by drastic methods, from the cryoglobulin studied at 22 degrees C, whereas no GM3 was extracted from two control cryoglobulins. Some minor gangliosides (representing less than 25% of total amount of bound gangliosides) were also extracted from Mou cryoglobulin and these gangliosides were shown to crossreact with GM3, as they specifically bind to Mou cryoglobulin by ELISA. After cryoprecipitation the serum still contained a monoclonal anti-Pr2 IgM kappa. A GM3-like ganglioside could be extracted from this purified IgM, and cryoprecipitability could be induced by the addition of a minute amount of micellar GM3. These results suggest that Mou cryoglobulin circulates as an immune complex and that cryoprecipitation may depend on unique IgM-GM3 (or IgM-GM3 cross-reacting gangliosides) complexes.