The mechanisms of suppression of tumor growth by the immune system will be studied. Two elements that mediate the specific recognition of antigens on tumor cells are humoral antibody and immune T-lymphocytes. This recognition subsequently leads to the destruction of the tumor cells, mediated primarily by elements other than antibody and immune T-lymphocytes, such as macrophages. We plan to investigate how macrophages function in vivo and in vitro in the control of tumor growth. More specifically we will investigate: 1) How macrophages get to the site of tumor growth and become active in tumor cell killing. 2) What molecules, released by macrophages, inhibit the division of tumor cells, and 3) What molecules, released by macrophages, stimulate the division of lymphocytes.