A well-demarcated solitary splenic mass (20×20×15 cm in size) containing hemorrhagic and necrotic foci was observed in a 4-year-old Thoroughbred stallion. Histologically, the mass consisted of lymphoma cells of the diffuse large non-cleaved type, with a high mitotic index and scattered macrophages that formed a starry sky pattern. The lymphoma cells revealed diffuse positivity for acid phosphatase and alpha naphthyl butyrate esterase, and were also positive for intracytoplasmic IgM on occasion, and mostly for proliferating cell nuclear antigen. Ultrastructural examination revealed moderately-developed rough endoplasmic reticulum sometimes with dilated cisternae. Thus, the diagnosis was a primary splenic lymphoma of B cell origin, but the exact reason for the absence of invasive growth or metastasis despite the high proliferative activity of this neoplasm was unclear.