The effect of 10-hydroxy-2-decenoic acid (10-HDA), the unique component of royal jelly, on maturation and functions of human monocyte-derived dendritic cells (MoDCs) in culture was investigated. It has been shown that 10-HDA, at concentrations higher than 500 μM, induces apoptosis of MoDCs. A lower dose (50 μM) stimulated T helper (Th)1 and down-regulated Th2 immune responses, as judged by the levels of interferon-gamma (IFN-γ) and interleukin (IL)-4, respectively, in supernatants of 10-HDA-treated MoDCs cultivated with allogeneic CD4+T cells. In contrast, a higher dose of 10-HDA (500 μM), although non-cytotoxic, inhibited maturation of lipopolysaccharide (LPS)-stimulated MoDCs. Such treated MoDCs produced lower levels of IL-12, IL-18 and tumour necrosis factor-alpha (TNF-α) and down-regulated both Th1 and Th2 immune responses. In conclusion, our results suggest that 10-HDA exerts different activity on human MoDCs, depending on applied concentrations, which is important when considering its therapeutic immunomodulatory property.