A case of plasmacytoma of the pleural cavity is reported with massive malignant pleural effusion, which, most unusually, lacked monoclonal gammopathy, thereby making it difficult to distinguish from lymphoma. The pleural tumor and pleural effusion contained large mononuclear lymphoma-like cells with distinct nucleoli. Immunohistochemistry revealed neither lymphoma markers nor clonal cytoplasmic nor cell surface immunoglobulins. Tumor cells were stained with vimentin and the plasma cell markers, VS38c, CD138 (syndecan-1), and MUM1 antibodies. Bone marrow contained small amounts of tumor consisting of similar cells. Electron microscopy showed well developed rough endoplasmic reticulum and peripherally positioned nuclei with euchromatin. Flow cytometry of bone marrow revealed a minimal involvement of CD38-positive cells. Chromosomal analysis of marrow cells revealed a complex abnormal karyotype. A polymerase chain reaction demonstrated clonal re-arrangement of the immunoglobulin heavy-chain gene. The overall results indicate a clonal expansion of tumor cells with primitive plasma cell differentiation with the highly unusual feature of absent monotypic immunoglobulin. The study illustrates the need for a comprehensive array of techniques to distinguish such rare non-synthesizing and non-secretory plasmacytomas from lymphoma.