Prostaglandin (PG) D(2) is abundantly produced by mast cells in the sites of allergic inflammations and acts on various cell types through its receptors DP and CRTH2. Among human T cells, CRTH2 is preferentially expressed on Th2-type cells. However, distribution of DP among T cells and impacts of CRTH2- and DP-mediated signals on T cell functions are presently unclear. Here, we show that CD4(+) and CD8(+) T cells producing IFN-gamma and IL-2 were reduced by DP-mediated signals, while CRTH2-mediated signals enhanced IL-2, IL-4, IL-5, and IL-13 production by Th2 cells. CRTH2 signals also caused up-regulation of CD11b and CD40L in resting Th2 cells. RT-PCR analysis revealed distribution of DP among Th cell subsets. On CRTH2(+) Th2 cells, the CRTH2-mediated PGD(2) effects were dominantly observed. Thus, PGD(2) favors Th2 functions through CRTH2 while restraining Th1 functions via DP, which may contribute to development of Th2-dominated status in allergic inflammations.