Experimental design. Cell suspensions prepared from embryonic (wild-type or α4 integrin^–/–) tissues were cotransplanted with competitor wild-type BM cells intravenously into adult irradiated recipients. Primary recipients were analyzed using various approaches. Repopulation activity was assessed by 2 different (but complementary) criteria: (1) the number of recipient mice repopulated, and (2) the level of repopulation of individual mice (≥ 5%). Colonization capacity was assessed by donor repopulation of different recipient tissues. Differentiation capacity was assessed by the analysis of lymphoid and myeloid cell subsets in recipient hematopoietic tissues. Serial transplantations of α4 integrin^–/– BM cells also were carried out and the peripheral blood of secondary recipients analyzed for donor contribution.