Autologous extramedullary marrow implantation leads to the formation of a nodule consisting of bone and hemopoietic tissue. Irradiation of the marrow immediately prior to implantation with up to 1000 rads does not interfere with the regenerative process. Since this amount of radiation is lethal for most of the hemopoietic cells the cell initiating the regeneration must be more radioresistant and is therefore probably associated with the relatively radioresistant marrow stroma. When the marrow was irradiated with 4000 rads 2 wk prior to implantation it did not regenerate even though at the time of implantation it contained numerous hemopoietic cells. This suggests that the hemopoietic cells do not play a significant role in the regeneration of marrow implants.