The kinetics of growth and differentiation of hematopoietic stem cells differ markedly according to their origin. A study of the ability of CFU from bone marrow (BM) or spleen to repopulate hemopoietic organs has been carried out in lethally irradiated mice restored with BM cells admixed with spleen cells bearing different chromosomal markers. Hemopoietic cells originating from AKR (40 acrocentrics) and AKR/T1ALD (36 acrocentrics + 2 metacentrics) mice were engrafted into lethally irradiated (AKR X AKR/T1ALD)F1 or (C3H X AKR/T1ALD)F1 hybrid recipients. Within 10 days, the BM-derived elements outnumbered the spleen-derived population in BM and spleen. This held even when the number of injected spleen-CFU was twice that of BM-CFU. This difference of growth rate subsided within 20 days. The first cells to reappear in the thymus bore the recipient karyotype (endoregeneration); they were later replaced by BM-derived elements but spleen-derived cells were never present in thymus in the case of competitive engraftment. In contrast, the lymph node cells bore the BM karyotype as well as the spleen karyotype. Injecting the spleen cells 3 days prior to the BM cells partially counterbalanced the over-growth of the BM-derived elements in the BM and spleen but did not affect the thymic repopulation which remained strictly derived from BM-CFU. When mice were injected only with BM-CFU or only with spleen-CFU, BM-derived cells were found in the thymus as early as 10-12 days after engraftment whereas the spleen-derived cells did not appear in the thymus until days 18-20.