Presence of A+U-rich elements (AREs) within 3'-untranslated regions (3'UTRs) of numerous mRNAs has been associated with rapid mRNA turnover; however, the interaction of specific factors with AREs is also associated with mRNA stabilization. Recently, two ARE binding proteins with putative mRNA destabilizing (AUF1) and stabilizing (HuR) properties have been described. However, no direct comparison ofAUF1 and HuR binding properties has been made. Therefore, we examined the relative affinities of p37AUF1 and HuR for a diverse set ofARE-containing mRNAs encoding beta-adrenergic receptors, a proto-oncogene, and a cytokine. We find that high-affinity AUF1 binding appears to require elements beyond primary nucleotide sequence. In contrast, binding of HuR appears considerably less constrained. As a functional correlate, we determined the ability of these specific mRNA sequences to affect the stability of chimeric beta-globin mRNA constructs. Although the relative affinity ofAUF1 and HuR are generally predictive of mRNA stability, we find that certain mRNA sequences do not conform to these generalizations.