Two models were employed to demonstrate the role of the thymus and T cells in antigen-induced lymphocyte trapping: 1) A graft-vs-host response (GVHR) of parental thymocytes to the antigens of lethally irradiated F1 mice; 2) Challenge of T cell-deprived and reconstituted mice with sheep erythrocytes or keyhole limpet hemocyanin. In both experimental situations T cell-dependent augmented localization of 51Cr-labeled cells was demonstrable in the lymphoid tissues. Adult thymectomy and/or irradiation without T cell supplementation diminished, but did not always abrogate, the trapping response. In the GVHR, trapping preceded the peak DNA synthetic response of the parental thymocytes, and correlated with the dose of injected cells.