Royal jelly (RJ), especially its protein components, has been shown to possess immunomodulatory activity. However, almost nothing is known about the influence of RJ fatty acids on the immune system. In this work we studied the effect of 10-hydroxy-2-decanoic acid (10-HDA) and 3,10-dihydroxy-decanoic acid (3,10-DDA), isolated from RJ, on the immune response using a model of rat dendritic cell (DC)-T-cell cocultures. Both fatty acids, at higher concentrations, inhibited the proliferation of allogeneic T cells. The effect of 10-HDA was stronger and was followed by a decrease in interleukin-2 (IL-2) production and down-regulation of IL-2 receptor expression. Spleen DC, cultivated with 10 microg/ml of fatty acids down-regulated the expression of CD86 and the production of IL-12, but up-regulated the production of IL-10. In contrast, DC, pretreated with 100 microg/ml of 3,10-DDA, up-regulated the expression of CD86 and augmented the proliferation of allogeneic T cells. The highest dose (200 microg/ml) of both fatty acids which was non-apoptotic for both T cells and DC, down-regulated the expression of MHC class II and CD86, decreased the production of IL-12 and made these DC less allostimulatory. The immunosuppressive activity of 3,10-DDA was also confirmed in vivo, using a model of Keyhole lymphet hemocyanine immunization of rats. In conclusion, our results showed the immunomodulatory activity of RJ fatty acids and suggest that DC are a significant target of their action.