Toll-like receptors (TLRs) constitute a family of nonpolymorphic receptors that are devoted to pathogen recognition. In this work, we have explored the impact of TLR ligands (TLR-L) on human hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs). We show that HSCs and HPCs have a comparable pattern of expression of TLR transcripts characterized by the predominance of TLR1, -2, -3, -4 and -6. In longterm cultures of HSCs, HPCs and stromal cells, most TLR-L profoundly inhibited B-cell development while preserving or enhancing the production of myeloid cells. In short-term cultures, the TLR1/2 ligand PAM 3 CSK 4 induced a large proportion of HPCs to express markers of the myelomonocytic lineage. PAM 3 CSK 4 induced only marginal expression of myeloid lineage markers on HSCs but promoted their myeloid commitment as revealed by their acquisition of the phenotype of multi-and bipotential myeloid progenitors and by upregulation of the transcription factors PU.1, C/EBPa and GATA-1. Our results suggest that TLR agonists can bias the lineage commitment of human HSCs and shift the differentiation of lineage-committed progenitors to favor myelopoiesis at the expense of lymphoid B-cell development.