Pluripotent hematopoietic stem cells from the bone marrow of newborn mice (9--13 days of age) have been studied following the administration of hydroxyurea (HU, 1,000 mg/kg). Following a single injection as well as a regimen of eight injections complete bone marrow reconstitution was achieved after 7 days. During the recovery phase diffusion chamber progenitor cells (DCPC) from pretreated newborn mouse bone marrow proliferated strongly to give significantly higher cell yields than DCPC from control animals. By contrast (colony-forming units-spleen [CFU-S]) levels were reduced at the same time to between 15 and 40% of controls suggesting a small-sized reserve pool of resting CFU-S in newborn mice. Assuming that DCPC are less differentiated than CFU-S and are non-cycling stem cells they may possibly pass through the CFU-S stage rapidly in newborn mice under the stress of hematopoietic reconstitution and proceed directly to myelopoiesis.