Abstract Intravenous (i.v.) immunization of mice with irradiated (2000 rads) allogeneic lymphoid cells induces the generation of suppressor T cells. Such suppressor T cells can suppress the anti-host delayed type hypersensitivity (DTH) by other T cells during acute Graft-versus-Host reactions in irradiated recipient mice. Suppression of anti-host DTH is detectable at least 50 days after i.v. induction of the state of suppression in the donors. The generation of suppressor T cells in the donors is associated with proliferation. Furthermore, the suppressor T cells need to proliferate further in the irradiated allogencic hosts in order to display a maximal suppressive effect. Two types of suppressor T cells were found to be required for suppression of the anti-host DTH response: a Lyt-1 +2 − cell population and a Lyt-1 −2 + cell population.