We evaluated the immunohistochemical profile and specificity of CD138 reactivity in 238 specimens from hematopoietic and nonhematopoietic neoplasms. In 91 bone marrow biopsies, CD138 reactivity was observed for nonneoplastic plasma cells, neoplastic plasma cells in multiple myeloma cases (43/43), and the plasmacytic component in lymphoplasmacytic lymphoma cases (4/4). Stromal reactivity was noted in 7 multiple myeloma cases. Of 9 bone marrow specimens involved by metastatic carcinoma, tumor cells were CD138+ in 5 cases; stromal reactivity was noted in 7 cases. Studies of 76 nodal and extranodal lymphomas (B-cell, 49; T-cell, 8; Hodgkin lymphoma, 19), 1 Langerhans cell histiocytosis, and 14 nonneoplastic lymph nodes revealed CD138 reactivity only for nonneoplastic plasma cells, the neoplastic cells of 2 large B-cell lymphomas (immunoblastic type, plasmacytoid features), and the clonal plasmacytic component of 3 of 3 extranodal and 1 nodal marginal zone lymphoma. Evaluation of 56 epithelial and nonepithelial tumors revealed CD138 positivity for neoplastic cells of carcinomas of various types (30/33), frequently with associated stromal reactivity, and for neoplasms of mesenchymal, melanocytic, and other tumor types (12/23). Within the hematopoietic system, CD138 is an excellent marker of plasmacytic differentiation. Based on its broad staining profile, CD138 reactivity for neoplastic cells is not a definitive marker for plasmacytic derivation, unless a hematolymphoid origin has been established.