A technique of irradiating the entire mouse except for one hind limb was developed to provide repeated proliferative demand on the stem cell pool. Animals received 200 cGY weekly for a total dose of 3,400 to 4,000 cGy. During irradiation, shielded bone marrow cellularity was similar to that of unirradiated controls. Shielded marrow colony- forming unit (CFUs) content increased while marrow CFUs self renewal capacity decreased as compared with unirradiated age-matched controls. Following irradiation experimental animals were monitored monthly for 10 to 12 months for marrow cellularity, CFUs content, and self renewal capacity. Shielded marrow cellularity and CFUs content remained elevated over age-matched controls throughout the period of observation. These findings are compatible with the requirement of the shielded hind limb to provide hematopoietic support for the remainder of the animal. Shielded marrow self renewal capacity, a measurement reflecting primitive hematopoietic stem cell function, remained depressed and did not recover with time. These experiments provide evidence for there being limitations on the self renewal capacity of the stem cell compartment. While the small amount of shielded marrow had sufficient capacity to support the animal its average self renewal capacity was permanently reduced.