The objective of this research program is to gain a better understanding of macrophages and their responses to tumors. An established cell line of mouse peritoneal macrophages (IC-21 macrophages), which possess cell-mediated immune reactivity, will serve as a macrophage model but relevant experiments will be repeated with macrophages from mice. The program is divided into two separate but related projects, the first of which will further define and characterize the immunoglobulin class-specific Fc receptors on macrophages and will relate their presence to events that occur as a result of recognition at those sites. The second project will utilize information obtained in the first project and will employ syngeneic tumor lines to relate specific anti-tumor responses of macrophages to specific cells. This project will exploit the ability to separate peritoneal macrophages into density-dependent subpopulations of cells that are functionally distinct. I will examine the ability of the subpopulations to be rendered anti-tumor effector cells by both immunologically specific and nonspecific means, and will attempt to relate any alterations in macrophage functions to specific subpopulations of cells where subtle yet significant differences can be detected.