During bacterial infection, the bone marrow hematopoietic activity shifts toward granulocyte production, which is critical for host defenses. Along with this enhancement of granulopoiesis, the bone marrow also increases its release of hematopoietic precursors. At the present time, little is known about the commitment of hematopoietic precursor cells, including hematopoietic stem cells and progenitors, in this response. To investigate the hematopoietic precursor cell response to bacterial infection, bacteremia was established in Balb/c mice by i.v. injection of Escherichia coli. Bacteremia caused a 10-fold increase in the number of lineage (lin)-c-kit+Sca-1+ cells in the bone marrow. This dramatic expansion of the lin-c-kit+Sca-1+ cell pool resulted from both increased mitosis of these cells and inversion from lin-c-kit+Sca-1- cell phenotype. Lipopolysaccharide, tumor necrosis factor-alpha, and interleukin-6 were potent factors capable of mediating phenotypic inversion of lin-c-kit+Sca-1- cells. Cells in the expanded lin-c-kit+Sca-1+ cell pool contained more colony-forming unit-granulocyte/macrophage. Mobilization of lin-c-kit+Sca-1+ cells into the circulation was significantly enhanced following bacteremia. These results demonstrate that the lin-c-kit+Sca-1+ cell population in the bone marrow constitutes a key component of the host defense response to bacteremia. Functional modifications of these primitive hematopoietic precursors are critical for enhancing granulocyte production following bacterial infection.