Monoclonal immunoglobulin component (M-component) presence is suspected based on serum protein analysis using cellulose acetate membrane electrophoresis, and finally clarified by determining its immunoglobulin class using immunoelectrophoresis (IEP) or immunofixation (IFE). M-component presence is essential for diagnosing multiple myeloma (MM) and primary macroglobulinemia; however, since it is also found in non-malignant conditions, called MGUS (monoclonal gammopathy of undetermined significance), the differentiation of MGUS from malignant diseases is often important. Bence Jones protein (BJP), once detected, can support the diagnoses of MM and primary AL-amyloidosis. In the latter condition, which is often difficult to diagnose, BJP is very helpful. The newly developed method measuring free immunoglobulin light chains can effectively indicate the presence of BJP in serum. The detection of BJP in urine is still important. Capillary electrophoresis combined with immunoabsorption can detect BJP in non-concentrated urine. It may be time to take such new methods into consideration in Japan.