ABSTRACT. A case of signet ring cell lymphoma in a 3-year-old mixed-breed sow is described. Macroscopical examination revealed enlargement of superficial, thoracic and abdominal lymph nodes and multiple tumor masses in the liver. The neoplastic tissue was composed of follicle center-like structures, in which neoplastic cells with Russell bodies were conspicuous. The bodies were immunostained for IgM (κ), and corresponded to moderately dense amorphous material within markedly distended cisternae of rough endoplasmic reticulum (RER) at the ultrastructural level. In contrast to typical signet ring cell lymphoma, the component cells of which resemble follicular center B lymphocytes with poorly developed RER, most neoplastic cells had features of plasma cells characterized by a cartwheel arrangement of heterochromatin and development of RER. Signet ring cells frequently had one or a few large Russell bodies occupying the entire cytoplasm, which may have been caused by abundant synthesis and defective secretion of immunoglobulin.