Prostaglandin E₂ (PGE₂) is known to be involved in cancer development and inflammation. E type prostanoid (EP) receptor are cognates for PGE₂ that have four main subtypes:EP1 to EP4. We have previously reported that PGE₂ activates the β-catenin/T-cell factor (Tcf) signaling pathway, which is known to contribute to the development of colorectal cancer, via EP4 receptor. Therefore, EP4 receptor-mediated signal transduction pathways are believed to be contribute to the development of cancer. On the other hand, prostaglandin D₂ (PGD₂) is known to have effects of cancer suppression and anti-inflammation. PGD₂ is a positional isomer of PGE₂, therefore, we investigated whether PGD₂ and its metabolites act on EP4 receptor and show anti-cancer effects. As a result, PGD₂ acted as a full agonist at least for the cAMP system of EP4 receptor, while PGD₂ hardly activated the β-catenin/Tcf signaling pathway. These results demonstrate that the one possible mechanism of anti-cancer effect of PGD₂ may result from this biased activity of PGD₂ on EP4 receptor.