We investigated the relationships between the serum γ-globulin concentration and the pathological characteristics in 4 types of bovine lymphosarcoma including the adult, calf, thymic and skin forms. In 5 cases of the adult form, there were extensive neoplastic proliferations in the lymph nodes and the γ-globulin concentration was low. Four cases of the calf form showed systemic lymph node involvement, and the concentration was low in one case and much lower in 3 cases. IgM was absent in 3 cases. In the thymic form, one case showed a slightly high concentration with systemic neoplastic lesions in the lymph nodes, whereas in the other the concentration was normal and lesions were localized. In a cow with the skin form, the concentration was low in the tumor stage but markedly increased in the regressive stage.