Accumulation of phenotypically-immature myeloid cells in the bone marrow of IL-2^−/− mice. Bone marrow cells from 6-week old wild-type (+/+) and IL-2^−/− (−/−) littermates were stained with biotin-labeled Mac-1 or Gr-1 antibodies followed by SA-PE and analyzed by flow cytometry. Note the reduction in the size of the mature granulocytes (Gr-1^hi) and myelomonocytic (Mac-1^hi) cell populations in IL-2^−/− mice. The level of staining obtained with isotype-matched control antibodies was used to determine the frequency of cells stained with Mac-1 and Gr-1 antibodies (% values shown in each histogram). The results shown are representative of eight independent experiments.