Lymphosarcoma-bearing dogs will be used as a model system for studying the nature of purified tumor-associated antigens and serum factors, including antibodies, that modify in vitro cell-mediated immune responses. Using various effector cell-target cell combinations, the quantitative cytotoxic reactivity of purified lymphocytes on radioisotope labeled target cultures of sarcoma cells or on antigen coated erythrocytes, will be analyzed. The influence of soluble antigens, antibodies, and antigen-antibody complexes on cell-mediated cytotoxicity will be investigated at the effector cell and target cell levels and specificity of the reactions will be determined. Monitoring of the humoral and cellular immune reactions in dogs with lymphosarcomas will be correlated with clinical stages of disease and will help to determine the effects of tumor growth or excision on a host's immune system.