The proposed research aims to develop the mouse mammary tumor system as an experimental model for cancer immunotherapy, to reveal fundamental aspects of antitumor immunity, and to indicate effective and hazardous immunotherapeutic procedures. Areas of investigation are primarily three: 1. To determine the antigenic strength, frequency of occurrence and interrelationship of virus-associated antigens and tumor-specific antigens in mammary tumor virus (MTV) infected and in MTV-free mice. The assay system will use standard sensitization-challenge methods. 2. To determine the effect on host resistance of specific immunotherapy in syngeneic and in autochthonous tumor hosts using killed tumor cell preparations and antigen-retaining RES organ preparations. The tests will compare the effects of route and vehicle of administration of different immunizing preparations, and the effects of timing and amount of preparation administered to hosts at different levels of specific resistance. 3. To study the variables in the immunization procedures which are critical in the stimulation of immune resistance factors while minimizing the risk of specific depression of host resistance by excess antigen or interference by blocking factors. The development of cellular and humoral immune factors will be assayed by passive transfer to normal or irradiated (400R) unsensitized recipients at the time when the recipiets are challenged with implants of living tumor cells. The challenge will use s.c. implantation and will also use intravascular implantation to study immune factors of importance under conditions simulating metastatic spread of tumor growth.