The CCAAT/enhancer-binding protein beta (C/EBP beta) transcription factor is required for granulopoiesis under stress conditions. However, little is known about its roles in steady state hematopoiesis. Here, we analyzed the peripheral blood and bone marrow of Cebpb(-/-) mice at steady state by flow cytometry and unexpectedly found that the number of peripheral blood monocytes was severely reduced, while the number of bone marrow monocytes was maintained. The ability of Cebpb(-/-) bone marrow cells to give rise to macrophages/monocytes in vitro was comparable to that of wild-type bone marrow cells. Apoptosis of monocytes was enhanced in the peripheral blood, but not in the bone marrow of Cebpb(-1-) mice. These results indicate that C/EBP beta is required for the survival of monocytes in peripheral blood. (C) 2015 Elsevier Inc. All rights reserved.