Leukotrienes (LT), products of the 5-lipoxygenase (5-LO) pathway, and their four G protein-coupled receptors (LTRs) participate in inflammatory tissue reactions and immune responses. Here we present preliminary evidence that LT may also contribute to endothelial cell differentiation in embryoid bodies (EBs) derived from the mouse embryonic stem cell line CCE. We observed differentiation-dependent LTR transcripts by real-time PCR (RT-PCR). Furthermore, 5-LO/CD68 double positive cells appeared during EB differentiation. To obtain preliminary evidence for a functional impact of the 5-LO/LTR system in EB differentiation, we used three distinct inhibitors of the 5-LO pathway: uBAY-9773 (combined CysLT1/CysLT2 antagonist), Montelukast (selective cysLT1 antagonist) and MK-0679 (LTD4 antagonist). All three inhibitors at 0.5–2 mM significantly decreased the endothelial cell marker PECAM-1 within six days. Thus, 5-LO may participate in angiogenesis of EBs.