B lymphocyte surface immunoglobulins (Smlg) were studied in 24 patients with multiple myeloma by means of anti-isotypic antisera, and their heavy and light chain isotypes were compared in each patient with those of the paraprotein. In 21 patients, lymphocyte Smlg consisted of only one light chain type, and in 16 of only 1 heavy chain type. However, the Smlg and paraprotein heavy and light chain types were identical in only 5 patients while in 6 they differed in heavy and light chain types, in 7 in light chain type, and in 4 in heavy chain type. In 2 patients with light chain myeloma, Smlg light chains were isotypically the same as the paraprotein. Isotypic discordance between paraprotein and Smlg may signify the proliferation of a second malignant clone with failure to differentiate into secreting plasma cells. Alternatively, it is conceivable that the lymphocyte Smlg could have the same idiotypic specificity as the paraprotein despite the isotypic differences, but this will require further studies using anti-idiotypic antisera.