Syndecan-1, an important transmembrane heparan sulphate proteoglycan is expressed in distinct stages of differentiation of normal lymphoid cells: in pre-B cells and Ig-producing plasma cells; however, its normal function, or presence in lymphoid malignancies, is still largely unknown. The expression of syndecan-1 (CD138) was studied in 57 human non-Hodgkin lymphomas using immunocytochemistry and immunohistochemistry. Positive expression of syndecan-1 was found in the plasma cells in chronic lymphoblastic leukaemia (B-CLL) cases, in different plasmocytoid lymphomas as well as in myeloma. All normal and malignant Tcells, or CD5+ cells other than B-CLL proved to be negative. These results strongly suggest that syndecan-1 expression is a characteristic phenotypic marker for B-CLL and lymphoplasmocytoid lymphomas and could be used for diagnostic purposes.