SummaryRetransplantation of bone marrow was studied in an attempt to understand why foetal liver was superior to adult bone marrow in the treatment of lethally-irradiated mice.Presensitization experiments, in which recipients were immunized against cells of the future donor-strain, were performed. In this system foetal-liver-chimeric spleen-cells were as effective antigenically as those taken from adult-bone-marrow chimeras.Foetal-liver-chimeric bone-marrow, used to protect irradiated mice that had been presensitized to normal homologous donor-type cells, was as ineffective as that from normal mice of comparable strain. From these results an unaltered sensitivity of the foetally-derived chimeric cells to specific isoantibodies was deduced.The findings are discussed in relation to several postulates offered to explain the ameliorating effect of foetal liver on homologous disease in radiation chimeras. Investigations presented here test, but the results fail to support, a loss of either isoantigenicity or s...