CD138, syndecan-1, is a transmembrane proteoglycan with a main cellular expression in stratified and simple epithelia. Within the hemopoietic system, CD138 is mainly confined to late stages of B-cell differentiation. CD138 expression is reduced during malignant transformation of various epithelia, and CD138 is rapidly shed by myeloma cells entering into apoptosis. This antibody is a useful aid for classification of multiple myeloma. Anti-CD138 may also be useful for the subclassification of diffuse large B-cell lymphomas.