Extramedullary plasmacytomas are uncommon neoplasms in dogs. Some tumors of this type may have been classified as reticulum cell sarcomas or reticulosarcomas. Reticulum cell sarcoma is a nonspecific term which encompasses large lymphoid cell tumors, extramedullary plasmacytomas, and true histiocytic (mononuclear phagocytic) tumors. Careful morphological assessment of both light microscopic and ultrastructural features and the use of immune markers usually distinguish the different subclassifications of reticulum cell ~arcoma.~ Previously published reports of canine extramedullary plasrnacytomas include several subcutaneous tumors and one each in the stomach, colon, and gingi~a.~-~J Histologically these tumors consisted of sheets and cords of somewhat pleomorphic to well-differentiated plasma cells embedded in fibrovascular stroma. This paper describes two additional canine oral extramedullary plasmacytomas.A 9-year-old spayed female German shepherd dog was referred to the Texas A&M University Veterinary Teaching Hospital with a soft fluctuant mass, about 2 cm in diameter, in the right lateral oropharynx. Radiographic examination revealed a soft tissue mass in the pharynx with no evidence of bone destruction. Clinical pathologic parameters were within normal limits. The mass was surgically excised and submitted in 10% neutral buffered formalin for histopathologic examination.A 10-year-old male mixed-breed dog was referred to the hospital for a firm, red, lobulated gingival mass which had been present for 6 months and had grown rapidly during the month prior to presentation. Physical examination revealed a 3.0 x 2.2 x 3.0 cm mass on the mandibulargingiva, rostral to the canine teeth. The mass had displaced most of the lower incisors. The radiographic appearance was that of a soft tissue mass causing lysis of the rostral mandible. Clinical pathologic parameters were within normal limits. A partial mandibulectomy was performed, and the rostral mandible with the attached mass was submitted in 10% neutral buffered formalin for histopathologic examination.Approximately 5 pm sections of selected paraffin-embedded tissues were stained with hematoxylin and eosin (HE) and Congo red. Specimens including bone were decalcified in formic acid prior to embedding. Additional specimens from the second patient were transferred to Trump's solution, post-fixed in 1% osmium tetroxide, and stained 'en bloc' with 2% uranyl acetate. Thin sections were stained with uranyl acetate and lead citrate for ultrastructural examination. Histologically, both masses consisted of nests, sheets, and cords of round cells in a fine, well-vascularized fibrovascular network. Spaces containing eosinophilic fluid and free cells were present throughout the first mass. Most cells in the first mass were 8 to 9 pm in diameter and well-differentiated, often with prominent Golgi halos while those in the second mass were larger and more pleomorphic (Fig. 1). In both masses nuclei were eccentric, round, basophilic, and stippled with dense chromatin. The finely gr...

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