Post-transcriptional gene regulation plays an important role in the expression of granulocyte-macrophage colony-stimulating factor (GM-CSF). Cytokine secretion by activated lymphocytes or mast cells is preceded by dramatic stabilization of the normally labile GM-CSF mRNA. The 3'-untranslated region of GM-CSF and other labile mRNAs contain the destabilizing motif adenosine-uridine-uridine-uridine-adenosine (AUUUA). We recently identified a cytoplasmic protein denoted the adenosine-uridine binding factor (AUBF) which binds with high affinity and specificity to AUUUA elements in synthetic RNA transcripts. We now demonstrate that AUBF binds specifically to GM-CSF mRNA through the destabilizing AUUUA elements. The formation of AUBF-GM-CSF RNA complexes required calcium or magnesium which were sensitive to EDTA or EGTA. A variety of other divalent metals blocked magnesium-dependent AUBF activity. These observations suggest that AUBF may protect GM-CSF mRNA from rapid degradation and play a crucial role in the expression of cytokine genes.