Information was sought on the reactivity of thoracic duct lymphocytes (TDL) from parental strain mice injected intravenously with large numbers of irradiated semiallogeneic spleen cells. TDL collected at 1 day after spleen cell injection were almost totally depleted of lymphocytes able to produce cell-mediated lympholysis (CML), a graft-versus-host (GVH) reaction, and skin allograft rejection against the H-2 determinants on the injected spleen cells. Normal or near normal responses were observed against third-party determinants. In the case of CML, there was no evidence that the unresponsiveness was due to suppressor cells. In marked contrast, the capacity of TDL to exert a specific mixed lymphocyte reaction (MLR) against the injected determinants was reduced by no more than two to fourfold; this applied whether MLR were measured in vivo or in vitro. Injection of normal rather than irradiated semiallogeneic spleen cells gave similar results. Complete and specific removal of MLR-producing lymphocytes was achieved, however, in a different system in which parental strain T cells were filtered from blood to lymph through irradiated F1 hybrid mice. Since this system presumably provided a much higher concentration of H-2 determinants to the responding lymphocytes, it is suggested that the differing results obtained with these two systems may indicate that certain cells reactive to H-2 determinants are of low affinity, their reactivity being detected in the MLR, but not by other parameters. With both systems, MLR-producing lymphocytes reappeared in the lymph after 2-3 days; the cells collected at this stage gave an MLR of altered kinetics. The present data, in toto, suggest that under certain conditions of antigen presentation, virtually all recirculating lymphocytes reactive to a given set of H-2 determinants can be induced to leave the circulation for a period of 1-2 days. After responding to the injected determinants (presumably in organs such as the spleen), the cells re-enter the circulation in an activated state after 2-3 days.