Cyclooxygenase-2 (COX-2) is an early response gene induced in renal mesangial cells by interleukin-1β (IL-1β). The 3*-untranslated region (3*-UTR) of COX-2 mRNA plays an important role in IL-1β induction by regulating message stability and translational efficiency. The first 60 nucleotides of the 3*-UTR of COX-2 are highly conserved and contain multiple copies of the regulatory sequence AUUUA. Introduction of the 60-nucleotide sequence into the 3*-UTR of a heterologous reporter gene resulted in a 70% decrease in reporter gene expression. Electrophoretic mobility shift assays (EMSAs) demonstrated that mesangial cell nuclear fractions contain a multimeric protein complex that bound this region of COX-2 mRNA in a sequence-specific manner. We identified four members of the protein-RNA complex as HuR, TIA-1, TIAR, and the heterogeneous nuclear ribonucleoprotein U (hnRNP U). Treatment of mesangial cells with IL-1β caused an increase in cytosolic HuR, which was accompanied by an increase in COX-2 mRNA that co-immunoprecipitated with cytosolic HuR. Therefore, we propose that HuR binds to the proximal region of the 3*-UTR of COX-2 following stimulation by IL-1β and increases the expression of COX-2 mRNA by facilitating its transport out of the nucleus.