Prostaglandins (PGs) are lipid mediators derived from arachidonic acid by several enzymes including cyclooxygenase (COX)-1 and COX-2. We have previously shown that PGE2 regulates immune responses, such as Th1 cytokine production and T-cell proliferation, in cattle. However, it is still unclear whether other PGs are involved in the regulation of immune responses in cattle. Here, immunosuppressive profiles of PGs (PGA(1), PGB(2), PGD(2), PGE(2), PGF(1 alpha) and PGF(2 alpha)) were firstly examined using bovine peripheral blood mononuclear cells (PBMCs). In addition to PGE(2), PGA(1) significantly inhibited Th1 cytokine production from PBMCs in cattle. Further analyses focusing on PGA(1) revealed that treatment with PGA(1) in the presence of concanavalin A (con A) downregulated CD69, an activation marker, and IFN-gamma expression in both CD4(+) and CD8(+) T cells. Sorted CD3(+) T cells stimulated with con A were cultivated with PGA1, and IFN-gamma and TNF-alpha concentrations decreased upon PGA(1) treatment. Taken together, these results suggest that the treatment with PGA(1) in vitro inhibits T-cell activation, especially Th1 cytokine production, in cattle.