Abstract Previous studies using loss-of-function mutants revealed that CCAAT/enhancer-binding protein α (C/EBPα) and PU.1 are potential regulators for hematopoietic stem cells (HSCs). To gain further insight into the HSC regulation by C/EBPα or PU.1, we used transgenic mice expressing conditional forms of these transcription factors to examine whether their activation alone is sufficient for modulating HSC functions. The activation of C/EBPα or PU.1 in HSCs in vitro or in vivo led to their suppression of growth, decreased mixed colony formation, and impaired competitive repopulating activities because of their defective self-renewal. These effects were more prominently observed when C/EBPα was activated, and the differentiation capacity to megakaryocytic lineage was selectively impaired upon C/EBPα activation. Unexpectedly, the expression of Bmi-1 and HoxB4, well-known regulators for self-renewal of HSCs, was not affected by the activation of C/EBPα or PU.1, suggesting that they regulate HSC function through an as yet unknown mechanism. Our data suggest that the activation of C/EBPα or PU.1 is sufficient to repress stem cell capacities in HSCs, and their fine-tuned regulation is critical for HSC homeostasis. Disclosure of potential conflicts of interest is found at the end of this article.