Bone marrow biopsies from 42 patients representing a variety of paraproteinemias, including plasma cell myeloma, heavy chain disease, and light chain disease, Waldenström's macroglobulinemia, and primary amyloidosis, were examined by electron microscopy. No consistent association between the various morphologic types of plasma cells or occurrence of lymphocytoid plasma cells (transitional, intermediate form) and immunologic type of myeloma or other paraproteinemia was encountered. Similarly, no correlation between clinical stage of the disorder, i.e., lanthanic (asymptomatic), incipient, or advanced, and cell type was observed. On the other hand, a rare plasma cell with Russell bodies was encountered in the marrow specimen from only one patient who had Bence Jones proteinuria. The presence of Russell bodies in cells from patients who had paraproteinemias without such proteinuria suggests that in the latter the bodies may represent stored or accumulated light chains which are not being utilized in the biosynthesis of the complete globulin molecule. No unusual cytoplasmic inclusions were found in the cells from patients with paraproteinemia. Cytoplasmic particles which resembled virus-like structures, found in cells from some patients with light chain disease and myeloma, appear most likely to represent incipient or early stages in the development of Russell bodies. Intranuclear inclusions observed in some plasma cells from these patients resulted from entrapment of cytoplasm within nuclear folds.