Bone marrow cells were isolated from the humerii of C57BL/6J mice following a 13-day flight on the space shuttle, Space Transportation System (STS)-118 to determine how space flight affected differentiation of cells in the granulocytic lineage. We used flow cytometry to assess the expression of molecules that define the maturation/activation state of cells in the granulocytic lineage on three bone marrow cell subpopulations. These molecules included Ly6C, CD11b, CD31 (PECAM-1), Ly6G (Gr-1), F4/80, CD44 and c-Fos. The three subpopulations were small agranular cells (R1), larger granular cells (R2) which were mostly neutrophils and very large, very granular cells (R3) which had properties of macrophages. Although there were no composite phenotypic differences between total bone marrow cells isolated from Flight- and Ground-control mice, there were subpopulation differences in Ly6C (R1 and R3), CD11b (R2), CD31 (R1, R2 and R3), Ly6G (R3), F4/80 (R3), CD44 high (R3), and c-Fos (R1,R2 and R3). In particular, the elevation of CD11b in the R2 subpopulation suggests neutrophil activation in response to landing. In addition, decreases in Ly6C, c-Fos, CD44^high and Ly6G and an increase in F4/80 suggest that the cells in the bone marrow R3 subpopulation of space flight mice were more differentiated compared to the ground controls. The presence of more differentiated cells may not pose an immediate risk to immune resistance. However, the reduction in less differentiated cells may forbode future consequences to macrophage production and host defenses. This is of particular importance to considerations of future long-term space flights.