Waldenström's macroglobulinemia. Dense fibrillar and globular material (arrows) fills dilated cisternae of rough endoplasmic reticulum in a bone marrow plasma cell. By light microscopic immunoperoxidase, these inclusions contained IgM-κ, the same monoclonal Ig that was detected in the patient's serum. N, nucleus; M, mitochondria. (Original magnification × 12,500.) (Courtesy of Ann M. Dvorak, MD, Department of Pathology, Beth Israel Hospital, Harvard Medical School, 330 Brookline Ave, Boston, MA 02215.)