BACKGROUND We report a case of a multiple myeloma with light polymerized kappa chains, a phenomenon rarely described. METHODS Capillary electrophoresis of serum proteins (Capillarys 2 Flex Piercing Sebia ®) revealed the presence of two migrant monoclonal peaks in the gamma globulin area and identified by immunosubtraction (IS) as light chain (LC) κ. These results suggest either the presence of a LCκ monoclonal IgD or IgE or the presence of a biclonal gammopathy or finally a LCκ polymerization. Serum immunofixation (IF) was carried out, after serum depolymerization with beta mercaptoethanol (BME), using as antiserum anti IgD, anti IgE and anti LCκ total and free LC κ. Capillary electrophoresis and IS were also repeated after serum treatment with BME. RESULTS The depolymerization of our patient's serum showed the transformation of the two serum peaks into a single peak at the level of the κ light chains on both capillary electrophoresis and immunotyping techniques (IF and IS). CONCLUSIONS Polymerization of kappa light chains is a rare phenomenon which can cause difficulty in the interpretation of the serum protein electrophoresis and immunotyping.