Selected populations of thymus-derived (T) rat lymphocytes having specific immunological reactivity to chosen histocompatibility (H) alloantigens are found among the cellular products of the mixed lymphocyte interaction (MLI). Such specific selection seems to depend on (a) the antigen-induced proliferation of specific H antigen reactive cells (HARC), and (b) the disappearance of nonreactive cells from the cultures. When the surviving cells from this lymphocyte-antigen interaction are transferred into thymectomized, X-irradiated, marrow-reconstituted syngeneic recipients (B rats) which lack detectable T-lymphocyte functions, the lymphocyte populations subsequently recovered from the hosts possess the capacity to react in the MLI and in the graft-vs.-host (GVH) reaction, and the reactions have specificity for the original priming alloantigens. In addition, these findings identify the cell that reacts in the MLI with the GVH reactive cell.