The role of the macrophage as effector cells in tumor immunity is to be studied in the light of the functional heterogeneity of these cells. Subclasses of rat immune peritoneal exudates, obtained by density gradient centrifugation, will be examined for their cytotoxic activity against a Gross lymphoma (C58NT)D which is maintained and grown in tissue culture. Specific activation of macrophages by immune lymphocytes will be attempted by co-culturing immune lymphocytes with peritoneal exudate macrophages or subclasses of macrophages from non-immune rats. These specifically armed macrophages will be compared with peritoneal exudate macrophages that have been "activated" nonspecifically, in vivo or in vitro, with various agents, such as C. parvum and BCG vaccines as well as beryllium salts. The ability of immune anti-tumor serum to block the cytotoxic activity of specifically and nonspecifically activated macrophages will also be determined. In addition to investigating cell-mediated destruction of tumor cells by peritoneal macrophages, other sources of macrophages, spleen and lungs, will be assayed. A procedure employing Cr51 release will be used to assay cytotoxicity. The methodology to be used in tissue culture and cell separations is that previously employed by us or others in related studies.