After transplantation of lymphoid cells into lethally irradiated (semi)allogeneic mice specific anti-host directed effector T cells are generated. This can be demonstrated using a delayed type hypersensitivity (DTH) assay. In H-2 compatible combinations, Mls-locus antigens, but no other minor histocompatibility antigens, can induce the generation of such effector T cells. This paper shows that maximal anti-host DTH responses are obtained when the lymphoid cells transplanted constitute of a mixture of long-lived, recirculating T2 cells and short-lived, sessile T1 cells. It was demonstrated that anti-Mls locus-directed DTH effector T cells are the progeny of T2 cells, and that T1 cells amplify this response. The latter, however, are by themselves incapable of displaying anti-Mls DTH reactivity. The T1 cells were found to be of the Lyt-1+2+ phenotype, and the T2 cells of the Lyt-1+2- phenotype. The same Lyt phenotypes were found for T1 and T2 cells synergizing in the GvH reaction against H-2 alloantigens.