The rapid progress in the elucidation of antibody structure and function had a major impact on our understanding of monoclonal immunoglobulins. It appears that monoclonicity is a basic immunologic phenomenon, closely related to specificity. The amplification of this phenomenon, observed in pathological conditions such as the various plasma cell dyscrasias, has not been fully understood due to the lack of identifiable antigens capable of triggering this response. The functional and structural data accumulated from animal experiments and the clinical investigation of anti-dextran, anti-phosphorylcholine, anti-thioflavine and homogeneous anti-arsonate antibodies as well as of cold agglutinins., point to the presence of a common structure/function relationship in these molecules. Structural and serologic data, obtained from homogeneous antibodies isolated by us from the serum of patients with cancer of the colon and of the urinary bladder, revealed significant structural similarities expressed by HVR homology and cross-idiotypic specificity.