An important control point in gene expression is at the level of messenger RNA (mRNA) stability. The mRNAs of certain regulatory cellular proteins such as oncogenes, cytokines, lymphokines, and transcriptional activators are extremely labile. These messages share a common AUUUA pentamer in their 3' untranslated region, which confers cytoplasmic instability. A cytosolic protein was identified that binds specifically to RNA molecules containing four reiterations of the AUUUA structural element. This protein consists of three subunits and binds rapidly to AUUUA-containing RNA. Such protein-RNA complexes are resistant to the actions of denaturing and reducing agents, demonstrating very stable binding. The time course, stability, and specificity of the protein-AUUUA interaction suggests the possibility that the formation of this complex may target susceptible mRNA for rapid cytoplasmic degradation.