Leukotriene B4 (LTB4), derived from arachidonic acid, is a potent chemotactic agent and activating factor for hematopoietic cells. In addition to host defense in vivo, several eicosanoids have been reported to be involved in stem cell differentiation or proliferation. In this study, we investigated the effect of LTB4 on human cord blood CD34+ hematopoietic stem cells (HSCs). LTB4 was shown to induce proliferation of HSC and exert anti-apoptotic effect on the stem cells. Blockade of interaction between LTB4 and its receptor enhanced self-renewal of the stem cells. Effect of LTB4 on differentiation of CD34 + HSCs were confirmed by clonogenic assays, and induction of the expression of BLT2 (the low-affinity LTB4 receptor), during the ex vivo expansion was confirmed by reverse transcription? PCR. Our results suggest that LTB4-BLT2 interaction is involved in the cytokine-induced differentiation and ex vivo expansion of hematopoietic stem cells.