Hematopoietic stem cells, numbering approximately 1/100,000 cells in mammalian bone marrow, are capable of complete hematopoietic and immune reconstitution upon injection into a myeloablated host. The present study aimed to analyze the earliest events in reconstitution of lethally irradiated, host murine bone marrow and spleen, after injecting purified Thy 1(lo)Lin(-)Sca-1(+) stem cells. Thy-1(lo)Lin(-)Sca-1(+) cells were isolated by fluorescence-activated cell sorting (FACS) from the bone marrow of 4-week-old C57BL(Thy1.1, Ly5.1) mice and injected into preirradiated, syngeneic hosts. These stem cells were also injected into congenic hosts, i.e., C57BL(Thy1.2, Ly5.2), and confirmed the donor origin of hematopoietic cells in the reconstituted host mice. Hematologically stained smears of the spleen and bone marrow of stem cell-injected recipients were prepared at 11, 14, 17, 21, 24, and 28 days after stem cell injection, and nucleated erythroid cells, mature granulocytes, and their myeloid precursors, monocytes, and large and small lymphocytes were recorded as a proportion of all nucleated cells in each organ at each time interval. The results indicated that in the earliest post stem cell injection intervals, both organs were predominantly erythroid and myeloid. Only at the later intervals did both organs show high proportions of large lymphoid cells and their progeny, small lymphocytes. Thus, early (<1 month) dynamics of hematopoietic reconstitution after transplantation of purified hematopoietic stem cells, is cell lineage specific.