[en] The purpose of this investigation was to study the affects of low level x-irradiation on the development of the exogenous haematopoietic spleen colony. Supralethally irradiated recipient mice were injected with a known number of donor bone marrow cells and at various times after injection, exposed to 1.0 R. The primary objectives of the study were: (1) to determine if low dose x-irradiation affected the development of the haematopoietic spleen colony; (2) to determine if the affect was dependent upon the time of the low dose exposure following transplantation; and (3) to determine if the low level exposure had an affect on a particular cell line within the spleen colony. It was observed that an exposure to 1.0 R following donor bone marrow transplantation did not affect the ability of haematopoietic stem cells to form macroscopic spleen colonies. Colonies developing in recipient mice exposed to 1.0 R were larger than those developing in the spleens of recipient mice sham exposed to the low dose of x-rays. The level of cellular proliferation, as determined by ¹²⁵IUdR incorporation, was elevated to the colonies of the 1.0 R exposed animals. The data suggest that regeneration of the transplanted stem cells and their progeny began sooner in the recipient mice exposed to the low dose of x-rays than it did in the recipient mice not exposed to 1.0 R