A 12-year-old, female spayed Chihuahua was diagnosed with nonsecretory multiple myeloma on the basis of multiple osteolytic lesions, histological evidence of plasma cell infiltrate on a bone biopsy, and absence of a monoclonal protein on serum and urine electrophoresis. A 6-week course of prednisone therapy resulted in no clinical improvement and the dog was euthanized 2 weeks after presentation because of progressive neurological impairment. Bone marrow specimens were processed and stained for ultrastructural and immunohistologic evaluation. Staining with antisera to immunoglobulin (lg) G. IgM, and IgA was negative. Tumor cells in both the pelvic and rib masses displayed prominent reactivity with an antibody onsecretory multiple myeloma (NSM) is a rare N plasma cell dyscrasia that constitutes approximately 1 % of all cases of multiple myeloma in people. It is characterized by the absence of a monoclonal protein on serum and urine electrophoresis.' The ultrastructural and immunohistologic features of NSM have not been well documented in animals because of the rarity of this neoplasm and the limited availability of monoclonal antibodies specific for lymphocyte differentiation antigens of dogs. Ultrastructural features, including lack of cisternal dilatation, helped conf i r m the diagnosis of NSM in a previous report of a dog with histological evidence of plasma cell infiltrates and absence of typical secretory products in the blood and urine.2 Because of the morphological heterogeneity of lymphoproliferative neoplasms, immunohistochemical and ultrastructural studies will likely begin playing an invaluable role in the immunophenotyping and categorization of these disorders. The purpose of this report is to describe the clinical, pathological, and immunohistologic features of NSM in a dog. Pathology, Microbiology, and Immunology (Moore, Taylor) From the Departments of Molecular Biosciences (Marks) and Case ReportA 12-year-old, 4.8-kg, female spayed Chihuahua was referred to the Veterinary Medical Teaching Hospital (VMTH), University of California, Davis, for evaluation of progressive left hind-limb lameness of 6 weeks duration. Skeletal radiographs performed by the referring veterinarian (RDVM) revealed multiple osteolytic lesions in both femurs and the left iIium. Prednisone was administered by the RDVM at I mg/kg PO every other day for 4 weeks with no clinical improvement.Physical examination at the VMTH revealed a bright, alert, and responsive dog with normal vital signs. The abnormal physical findings at entry included left hind-limb lameness with pain on palpation of the left hemipelvis. A funduscopic examination and hemogram were unremarkable. Abnormal biochemical findings included increased serum alanine aminotransferase (ALT) activity of 282 IU/L (normal, I9 to 70 IU/L) and increased alkaline phosphatase (ALP) activity of 2,73 1 IU/L (normal, I5 to 127 IU/L). The serum globulin concentration was normal (3.8 g/dL). Urinalysis (antepubic cystocentesis) revealed a specific gravity of 1 .O 18, proteinuria (3+...

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