Individual B cells express either kappa or lambda light chains, but never both. In a polyclonal population the ratio of kappa to lambda bearing B cells is 2:1, and the occurrence of a mixture of kappa and lambda light chain-bearing cell types suggests polyclonality and a reactive or non-neoplastic proliferation of B cells. Diseases, such as multiple myeloma and B-cell lymphoma, are characterized by the proliferation of monoclonal neoplastic plasma cells producing only one type of light chain. Thus, the demonstration of a kappa or lambda light chain-restricted cell population is very useful in the histopathological diagnosis of these diseases. Primary AL (amyloid light-chain) amyloidosis is also a plasma cell disorder where the demonstration of monotypic kappa or lambda light chain in the amyloid deposits is diagnostically important and separates this disease from AA amyloidosis where immunoglobulin light chain deposits do not occur.