AbstractAthymic nude mice or thymectomized mice, irradiated and reconstituted with T‐ depleted bone marrow cells (“B mice”), were injected with allogeneic or syngeneic thymocytes bearing caryotypically distinct chromosomes. The fate of the thymocytes was investigated after various periods of time using two methods: (a) frequency of the cells bearing the Thy‐1 antigen as detected by immunofluorescence with a heteroantiserum among the recipient spleen cells, (b) presence of chromosomally detectable donor cells in phytohemagglutinin‐stimulated spleen cell cultures. These two methods indicate that allogeneic thymocytes disappear after about 7 days, semiallogeneic thymocytes after about 20 days, while syngeneic thymocytes, even when injected in 50 times lower number (106cells), are detectable for months, thanks to the sensitivity of the caryotypic method. Allogeneic thymocytes induce the production of high titers of alloantibodies, which were shown to react specifically against their own H‐2, a phenomenon interpreted as a “suicidal” allogeneic collaboration. These experiments demonstrate that the failure of allogeneic thymocytes, in contrast to syngeneic thymocytes, to achieve long‐term restoration of the immune responsiveness of T‐depleted mice is due to a rejection of the foreign thymocytes, and not to a failure of T and B cells to collaborate across the histoco