Transplantation tolerance induced by semiallogeneic cells in newborn mice or by the adoptive transfer of syngeneic spleen cells from neonatally tolerant donors in adult mice was studied in the strain combination with the H-2D region disparity (B10.A recipients-B10.A(2R) donors). Tolerance was transferred adoptively from 3-wk-old mice that had been rendered tolerant at birth; the ability for the transfer of tolerance persisted for long periods even when neonatally tolerant animals were not skin grafted. Neonatally and adoptively induced tolerance was not abolished by the adoptive transfer of 100 .times. 106 immunocompetent cells from normal syngeneic donors. In the in vitro experiments, cells from tolerant mice in the 2 types of tolerance reacted to the tolerated antigens in the mixed lymphocyte culture, but did not react to the tolerated antigens in the microcytotoxicity test (only some mice with adoptively induced tolerance showed a certain degree of reactivity). Cells from both types of tolerant mice inhibited the in vitro sensitization of cells from normal syngeneic animals. This suppression was stronger with cells from neonatally tolerant mice.