The graft-versus-host (GVH) response of spleen cells from rats bearing either orthotopic skin grafts or allogeneic implants in the anterior chamber of the eye was evaluated using popliteal lymph node (PLN) assay. When a viable implant remained in the anterior chamber, the spleen cells of these rats produced a popliteal lymph node enlargement in FI hybrids which was approximately 50% of that produced by a similar number of cells from a normal animal. Conversely, the GVH response of spleen cells from orthotopically skin-grafted rats was noted to be significantly increased over the response of spleen cells from normal animals. The decrease in the GVH response of implanted rat spleen cells was a specific reaction and not because of trauma or implantation, since spleen cells from rats bearing syngeneic implants had shown no reduction in their GVH-inducing ability. The PLN weights of rats receiving mixed population of normal and implanted rat spleen cells were always less than the weights observed with an equal number of normal spleen cells. These findings permit the assumption that implant-bearing rats may be lacking or low in cells that induce GVH reactions or that there is a delayed conversion of effector cells after early immune recognition.