We seek to characterize the mechanism of action of BCG-induced NK cells and to explore the possibility that this cell type may be linked to the therapeutic usefulness of BCG in causing regression of various neoplasms. Additionally, we have noted two other activities induced by BCG which may be germane to the regulation of host-tumor interactions. One of these, a suppressor cell(s), prevented both the primary induction of cytotoxic T cells and the differentiation of T memory cells in vitro. The other was a lectin-dependent cytotoxic cell. None of these effector activities were exhibited by cells from non-immunized animals, nor by peritoneal exudates induced with thioglycollate or proteose peptone. We will characterize these BCG-induced effector cells by studying their rate of appearance, mechanism of induction, specificity of effector function and mode of action. In addition we will use Winn-type assays to evaluate directly the roles of each of these effector cells in the BCG-induced immunotherapy of murine tumors. It is hoped that information from these studies will provide a more rational basis for intervention with BCG in human cancer.