The effect of murine anti-canine lymphoma monoclonal antibodies (MAbs) on tumor cell lysis by thioglycolate activated murine macrophages in vitro and tumor growth inhibition in athymic mice was studied. All IgG1 and IgG2a MAbs tested were able to promote specific destruction of canine lymphoma 17-71 cell line by activated macrophages. A correlation between higher ADCC activity and MAb isotype was not clearly evident. In vivo IgG2a and IgG1 MAbs inhibited the growth of canine lymphoma. These results suggest that MAbs of IgG type have potential in immunotherapy of dogs with lymphoma since they have high tumoricidal activity in vivo.