Lymph node and spleen cells from mice immunized in vivo to allogeneic of syngeneic liver antigen are cytotoxic for syngeneic liver cells, but not for syngeneic fibroblasts or established liver cell cultures of allogeneic origin. The cytotoxic activity is mainly dependent on T-cell activity, but a non-T-cell-mediated cytotoxicity may also play a role. Lymphocytotoxicity is inhibited by preincubation of the lymphocytes with syngeneic liver antigen, but not with syngeneic kidney homogenate. The liver-specific lymphocytotoxicity corresponds to the in vivo function of lymphocytes in the development of experimental hepatitis. In vitro immunization of lymphocytes in a Mishell-Dutton culture system also induces liver-specific cytotoxicity. The results indicate that the natural tolerance to self antigens can be lost after invivo as well as in vitro immunization. The induction of self-reactivity of lymphocytes in these experiments may be attributed to regulatory mechanisms of the immune reaction at a cellular level.