The expression of early and mature B cell markers, surface beta 2-microglobulin (B2M) and cytoplasmic immunoglobulin (clg) by aneuploid tumor cells in bone marrow aspirates from 44 patients with multiple myeloma was evaluated by correlated DNA immunofluorescence flow cytometry. Myeloma tumor cells of almost 90% of the patients contained monoclonal clg and expressed the mature plasma cell antigen R1-3 as well as surface B2M; common acute lymphoblastic leukemia antigen (CALLA) was present in 55%, B2 in 17%, and B4 in 23% of samples studied. Coexpression of CALLA and clg in 46% of all patients identified a novel myeloma phenotype without known counterpart in the normal differentiation of B cells. CALLA and clg were independently expressed and gave rise to CALLA+/clg-, CALLA+/clg+, and CALLA-/clg+ cells. The association of CALLA and mature plasma cell markers may define discrete stages of neoplastic plasma cell differentiation.