The nonspecific suppression of immunological responses that is generated within host popliteal lymph nodes upon exposure to syngeneic normal spleen cells has been examined. The suppression, which had previously been described as being capable of preventing initiation of cytotoxic T lymphocytes (CTLs) to hapten-altered self antigens, arises within 3 to 7 days after injecting the spleen cells. Suppression was shown to be attributable to an induced T cell that was functional when transferred intravenously. Although the cell surface marker(s) on both splenic B and T cells that stimulates appearance of Ts has not yet been identified, the cells possessing the marker were not required to be viable to cause the induction. We have shown here that the Ts is fully functional when it is put in the antigenic site used for CTL immunization. The induced Ts has been identified as bearing the Lyt 2.1 cell surface marker. Furthermore, it has been shown to be insensitive to cyclophosphamide (CY), thus differentiating it from the naturally occurring Ts cell (TS0) that is known to be CY sensitive. In addition to preventing induction of CTLs toward hapten-altered self antigens, exposing popliteal lymph nodes to syngeneic spleen cells induced Ts capable of suppressing the primary IgM antibody response to sheep red blood cells. The Ts cells that suppressed the primary antibody response possessed the same Lyt cell surface markers and CY insensitivity as the Ts that mediated suppression of the CTL response. Thus, evidence that two dissimilar immunological reactions may be down-regulated by the same suppressor mechanism has been provided. Results of a kinetic study showed that the Ts prevented development of both the humoral and the cell-mediated immune responses by affecting their inductive phases. Possible targets for suppression that more than likely would have to be common to the two widely different immune responses have been indicated.