SUMMARYHaematopoietic cells from BALB/c (H-2d) mice were serially transplanted in irradiated C57BL/Ks (H-2d) mice at intervals of 6–8 weeks. Two groups of recipients were established, one receiving and donating 50 × 106bone marrow and spleen cells, the other 5 × 106cells. The recolonization of the various groups of animals was followed by measuring the amount of donor-type (BALB/c) haemoglobin in the circulation of the recipients. In the first transfer full recolonization was observed in the recipients of 50 × 106or 5 × 106cells. However, in the second transfer, while the recipients of 50 × 106cells from the first stage recipients were fully recolonized by donor cells, the animals receiving 5 × 106cells abruptly ceased to form donor-type haemoglobin after initial recolonization. Full recolonization was again obtained in the recipients of 50 × 106cells in the third transfer, but donor-type haemoglobin was found only briefly and in very low levels in the recipients of 5 × 106cells. A slightly different pattern of recolonization was obtained when the recipients of 50 × 106cells were not bled before they had been used as donors for the next stage recipients. In this series of experiments rejection of the grafted tissue occurred one stage later than in the previous series of experiments. It is argued that the rejection of the grafted tissue cannot be due to the transfer of insufficient numbers of cells (total number). The possibility of whether this graft rejection is due to genetic differences at loci other than theH-2, or to the effect of a prolonged stimulus to divide produced by repetitive transfers of cells, is discussed.