Cytokine production is associated with both the normal and pathologic inflammatory response to injury. Previous studies have shown that the immunosuppressants cyclosporin A or FK506, which interact with the peptidyl-propyl isomerases cyclophilin A and FK506-binding protein (FKBP12), respectively, block cytokine expression. A third member of the peptidyl-propyl isomerase family, Pin1 is expressed by immune and other cells. Pin1 has been implicated in cell cycle progression, is overexpressed in human tumors, and may rescue neurons from tau-associated degeneration. However, the role of Pin1 in the immune system remains largely unknown. In this study, we analyze the role of Pin1 in GM-CSF expression by human PBMC and CD4+ lymphocytes. We show that Pin1 isomerase activity is necessary for activation-dependent, GM-CSF mRNA stabilization, accumulation, and protein secretion, but not non-AU-rich elements containing cytokine mRNAs, including TGF-beta and IL-4. Mechanistically, Pin1 mediated the association of the AU-rich element-binding protein, AUF1, with GM-CSF mRNA, which determined the rate of decay by the exosome.