Ten cases of bovine lymphohematopoietic neoplasms were investigated histologically, histochemically, and immunohistochemically, and were classified into eight histologic types on the basis of the origin and morphology of tumor cells. Case 1, a precursor B lymphoblastic leukemia, was positive for CD79a and terminal deoxynucleotidyl transferase. Case 2, a thymic B cell lymphoma, was also positive for these markers, but there were cytologic differences between the two cases. Cases 3-5 were diagnosed as pleomorphic B cell lymphomas, which were characterized by cytologic pleomorphism and expression of CD79a and CD5, and were etiologically associated with bovine leukosis virus (BLV). A case of diffuse large B cell lymphoma of the cerebrum (case 6) also showed a positive result for CD79a and CD5. However, the lymphoma was composed of a homogeneous population of large neoplastic cells, and was considered to be unrelated to BLV. The other B cell cases were categorized into immunoblastic (case 7) and lymphoplasmacytic (case 8) lymphomas, in which immunoglobulin-producing lymphoma cells were observed. In a cutaneous γδ T cell lymphoma (case 9), the neoplastic cells cytologically resembled those in case 6, but expressed CD3 and WC1. In case 10, an acute basophilic leukemia, some leukemia cells had intracytoplasmic granules that were metachromatic and tryptase positive but negative for naphthol AS-D chloroacetate esterase. Bovine lymphohematopoietic malignancies are classifiable into discrete histologic types according to immunophenotype. The classification is more scientific than the traditional one, the latter being based on the age of affected animals and/or the site of tumor formation.