It has been well established that bispecific antibodies containing anti-T-cell receptor MAbs crosslinked to anti-tumor MAbs induce T cells to lyse tumor cells, as measured in a 51Cr-release assay. Such lysis requires direct attachment between target and cytotoxic cells and most probably involves the exocytosis of cytolytic substances into the cell:cell interface. In addition, targeted T cells mediate a second activity, the secretion into the medium of factors that can block the growth of bound tumor cells and unbound bystander cells. In order to test how targeted effector cells mediate anti-tumor effects in vivo, we are currently developing a totally syngeneic murine system in which murine T cells are targeted against mouse mammary tumors. The system allows us to treat both primary tumors and tumor transplants, using a mammary-tumor-virus antigen as the entity that is specifically recognized on the tumor cells.