The mouse mammary tumor system will be studied with the following objectives: (1) To study the nature of the local immune reaction against mammary carcinoma MC1 in subcutaneous and intra-mammary implantation sites, and in the lungs after induced metastatic dissemination. The tests will use monoclonal antibodies against cell surface markers and peroxidase staining to identify the various effector cell types involved in the immune rejection response. (2) To study the supportive and the negative effects which may result when immunological treatments are given at different stages relative to primary tumor cure and secondary tumor development. The tests will use bacterial immuno-potentiators and killed tumor cells. (3) To study the effects on tumor growth and regressions when humoral and cellular tumor host factors are passively transferred to syngeneic athymic and normal tumor hosts. The tests will use whole blood and cells isolated from the primary stromal reaction against implants of MC1. (4) To search for a more relevant experimetal mammary cancer system by determining the viral or nonviral etiology of tumors from different C3H sublines.