IL-2^−/− bone marrow cells cannot sustain myelopoiesis in lethally irradiated host animals. Groups (n = 4 to 5) of irradiated host (B.6/CD45.1⁺) mice were injected with autologous (IL-2^+/+) bone marrow or with bone marrow from 5-week old gnotobiotic IL-2^−/− mice (IL-2^−/−), or IL-2^−/− bone marrow mixed with host bone marrow cells in 1:1 or 1:5 ratios. Approximately 30, 60, and 90 days postreconstitution, animals were bled and the mononuclear cells stained with either antibodies specific for the host or donor CD45 allele in combination with antibodies that identify the myeloid (Mac-1 and Gr-1; M&G), T (CD3/CD4/CD8) and B (B220) lineages and analyzed by flow cytometry. The results shown represent the levels (%) of donor-derived cells in each lineage at each time point for individual mice. The asterisk indicates that at 30 days postreconstitution the number of T cells was too low (<1%) to allow accurate quantitation.