The importance of the persistence of antigen for maintaining tolerance of transplantation isoantigens has been investigated. Established A strain skin grafts on CBA mice rendered tolerant at birth were destroyed by the passive transfer of C3H anti-A lymphoid cells. Evidence is presented that these C3H cells also eliminated cell chimerism and that they too were rejected. Inasmuch as such treated tolerant mice rejected subsequent A strain skin grafts, it is concluded that the destruction of these second grafts was mediated by immunologically competent cells of host genotype, i.e., cells which became competent as a result of the elimination of A strain antigens. This conclusion is supported by experiments in which tolerance was sustained in a high percentage of CBA mice who, shortly after the destruction of their original A strain grafts, received an inoculation(s) of CBA/A cells.