Vertebrate lectin purified from loach egg was tested for induction of tumor lysis mediated by macrophages. Loach egg lectin lysed tumor cells but not normal spleen cells in cooperation with BCG- or glucan (TAK)-elicited peritoneal macrophages of mice. Corynebacterium parvum-, OK432-, glycogen-, lipopolysaccharide-elicited or resident macrophages were not effective. Neither loach egg lectin nor BCG nor glucan macrophages alone had a cytolytic action on tumor cells. Thus, the vertebrate lectin from loach egg is a mediator in macrophage-mediated tumor lysis, inducing binding of macrophages to target cells. This lectin-dependent macrophage-mediated cytolysis (LDMC) was inhibited by galactose, N-acetylgalactosamine, fucose, or rhamnose. These results suggest that tumor cells can be recognized via glycoconjugates on cell membrane in addition to tumor-associated antigen and that some animal lectins participate in macrophage-mediated tumor lysis.