Murine lymphoid cells sensitized against allografts express two types of cytotoxicity measured in vitro in short-term assays of 2 to 3 hr by 51Cr release. One type of cytotoxicity is specific against target cells carrying the sensitizing alloantigens, and the other type is nonspecific against both syngeneic and unrelated target cells and requires the presence of phytohemagglutinin (PHA) or concanavalin A (ConA). Several lines of evidence are presented which demonstrate that both cytotoxic systems are mediated by the same clone of alloimmune effector T lymphocytes and that the lectin-induced cytotoxicity is not attributable to mitogenic polyclonal activation of effector cells. Studies with ConA inhibitor alpha-methyl-D-mannoside suggest a mechanism of interaction that requires the lectin to be present during the reaction. Experiments with other reagents, however, that bring about physical contact of the cells but not lysis, indicate that cytolysis can occur only through specific lectin receptors on the cells. The significance of this lectin-induced cytotoxicity is discussed.