Monoclonal IgG-containing type II cryoglobulins (CG) from 25 patients, most affected with autoimmune or inflammatory diseases, were characterized using a Western blotting procedure. Rheumatoid factors were measured by ELISA. Out of the 25 type II CG, 14 contained a single monoclonal IgG and 11 contained several distinct monoclonal Ig, as assessed by the electrophoretic mobility and isotypy of the narrow bands. Comparison of blots revealed by anti-IgG subclass antibodies in the patients' sera and CG showed a clear-cut enrichment of CG in both polyclonal and monoclonal IgG1 and IgG3. All monoclonal IgG in the CG belonged to the IgG1 or IgG3 subclasses. CG also contained more detectable RF of the IgM class than did the corresponding sera. Using conventional methods, the present CG either could not be typed or were all (immunoelectrophoresis) or often (immunofixation) classified as type III. The results challenge the classical classification of CG and show the necessity of the use of sensitive techniques for immunochemical characterization of mixed CG.