Previous studies have presented evidence of shared idiotypic antigenic determinants located within the variable (VH) region of the heavy chains of monotypic IgMlambda and IgGkappa isolated from the serum of an individual patient, Bro, with Waldenstrom macroglobulinemia. Comparative N-terminal VH sequence analyses have demonstrated that the respective micron and gamma chains belong to separate VH subgroups. The entire VH sequence of the Bro micron chain has been reported, but the VH sequence of the Bro gamma chain still awaits completion. We report the results of an immunofluorescent analysis of cytoplasmic Ig of lymphoid cells isolated from the patient's peripheral blood and bone marrow. Between 6% and 9% of the cytoplasmic Ig-positive lymphoid cells exhibited fluorescent evidence for the dual presence of kappa and lambda chains are well as micron and gamma chains. These results strongly suggest that the idiotypically related Bro IgMlambda and IgGkappa paraproteins are derived from a common clonal origin. Moreover, these findings extend the results of a previous study that has demonstrated the dual presence of IgGkappa and IgGlambda paraproteins within individual myeloma plasma cells. Collectively, these studies suggest that a single neoplastic lymphoid clone may not necessarily be restricted to the synthesis of Ig proteins of the identical light chain class. These findings may have a broad implication for the understanding of surface and cytoplasmic Ig markers of neoplastic lymphoid cells in certain other lymphoproliferative disorders.