A hybridoma-secreting monoclonal antibody, designated 2E12D5, was prepared by fusing mouse myelomas with spleen cells from a rat immunized with BCG-elicited mouse peritoneal macrophages. Binding of the antibody to primary mouse cells and cell lines was examined by indirect immunofluorescent flow cytometry. 2E12D5 was cytotoxic and of the rat IgG2a subclass. The antibody reacted to a great extent with the BCG-induced mouse peritoneal macrophages, half the bone marrow cells, macrophage-like cell lines and thioglycollate-induced peritoneal exudate cells to some degree, but not with other cells including BCG-elicited peritoneal lymphocytes, non- or low tumoricidal peritoneal exudate cells, thymomas, myelomas and fibroblasts. Immunoblot analysis showed the antibody to bind to four major proteins, 220,000, 125,000, 105,000 and 92,000 in molecular weight from the BCG-induced peritoneal macrophage. Pretreatment of BCG-induced peritoneal macrophages with 2E12D5 and rabbit complement greatly inhibited macrophage-mediated tumour cell cytotoxicity.