Histiocytic sarcoma was observed in a 12-year-old Japanese Black cow. Neoplastic lesions were detected in the bone marrow, and the spleen and lymph nodes were enlarged. Hemorrhagic foci were seen in a number of organs. Examination of the blood showed a leukopenia and a depressed platelet count. Histologically, neoplastic cells resembling mature histiocytes revealed sheet-like or diffuse growth pattern in the bone marrow, spleen and lymph nodes, and many showed erythrophagia in areas of severe hemorrhage. Immuno-positive reactions to lysozyme and HAM56 indicated that this neoplasm originated from histiocytes and was distinguishable from other lymphohematopoietic neoplasms with erythrophagocytosis.