Tetraparental bone marrow chimeras were produced by injecting lethally X-irradiated F1 hybrids with relatively high numbers of T-cell-depleted bone marrow cells from both allogeneic parental strains. The mice survival in excellent health and showed a stable, approximately 50:50 (parent:parent), lymphoid cell chimerism lasting for at least 7 mo after irradiation; regeneration of host-type hemopoietic cells was very limited. Thymus, lymph node, and thoracic duct lymphocytes showed specific unresponsiveness to host mixed leukocyte reaction (MLR) determinants. Similarly specific tolerance to H-2 antigens of host type was demonstrated in spleen and lymph node. No suppressor cells could be demonstrated in either system and blocking serum factors could not be found. The results suggest specific deletion of functional T cells reactive to host-type MLR and cell-mediated lympholysis determinants.