While benign (reactive) B-lymphocytic populations produce Ig-molecules containing an almost equal amount of kappa and lambda light chains, i.e. the number of cells producing kappa is more or less equal to the number producing lambda, neoplastic B-lymphocytic populations has light chain restriction (i.e., are monoclonal) producing either kappa or lambda. Consequently, demonstration of light chains is the most important procedure in the diagnosis of neoplasms of B-lymphocytes (lymphomas and leukemias). The amount of Ig produced in individual types of B-lymphocytic neoplasia vary and is often small, demanding a sensitive technique for detection. About 80% of non-Hodgkin lymphomas are B-cell lymphomas, of which the large majority express surface (s) IgM, although the expression in small cell lymphoma/chronic lymphocytic leukaemia is weak. Plasmacytoma/multiple myeloma do not show sIgM. Lymphoplasmacytic lymphoma most often shows sIgM, while plasmacytoma/multiple myeloma show sIgG in 50% and sIgA in 20%. Light chain restriction is the single most important marker for neoplasms of B-lymphocytic origin. Demonstration of heavy chains can sometime be of aid in the study of malignant lymphomas, as lymphoplasmacytic lymphomas are usually focally IgM positive, while plasmacytoma/multiple myeloma in most cases express strong cytoplasmic IgG or IgA. Precursor B-cell neoplasms are are Ig negative. Normal tonsil is appropriate control tissue: approximately 50% of the mantle zone B-cells should show a distinct membrane staining reaction, while the rest should be unstained.