The messenger RNAs of many proto‐oncogenes, cytokines and lymphokines are targeted for rapid degradation through AU‐rich elements (AREs) located in their 3′ untranslated regions (UTRs). HuR, a ubiquitously expressed member of the Elav family of RNA binding proteins, exhibits specific affinities for ARE‐containing RNA sequences in vitro which correlate with their in vivo decay rates, thereby implicating HuR in the ARE‐mediated degradation pathway. We have transiently transfected HuR into mouse L929 cells and observed that overexpression of HuR enhances the stability of β‐globin reporter mRNAs containing either class I or class II AREs. The increase in mRNA stability parallels the level of HuR overexpression, establishing an in vivo role for HuR in mRNA decay. Furthermore, overexpression of HuR deletion mutants lacking RNA recognition motif 3 (RRM 3) does not exert a stabilizing effect, indicating that RRM 3 is important for HuR function. We have also developed polyclonal anti‐HuR antibodies. Immunofluorescent staining of HeLa and L929 cells using affinity‐purified anti‐HuR antibody shows that both endogenous and overexpressed HuR proteins are localized in the nucleus. By forming HeLa–L929 cell heterokaryons, we demonstrate that HuR shuttles between the nucleus and cytoplasm. Thus, HuR may initially bind to ARE‐containing mRNAs in the nucleus and provide protection during and after their export to the cytoplasmic compartment.