To functionally classify AU-rich elements (AREs) from six different cytokine mRNAs, we made use of two previously described HT1080-derived cellular mutants (slowA, slowC) that lack a function required for the rapid degradation of interleukin-3 (IL-3) mRNA. Here we show that the defect is specific for ARE-containing mRNAs, whereas nonsense-mediated decay is intact. Degradation of β-globin reporter transcripts mediated by the AREs of IL-3, GM-CSF, and TNFα, as well as by the structurally different and less potent AREs of IL-2 and IL-6, is impaired in both mutants. All these reporter transcripts are also sensitive to decay induced by ectopic expression of the RNA-binding protein tristetraprolin in the slowC background. Thus, we concluded that the mutants slowA and slowC define a common mRNA degradation pathway that targets cytokine AREs. In NIH3T3 cells, this decay pathway becomes incapacitated by upstream signaling from p38 MAP- or PI3-kinases, which independently stabilize cytokine ARE-containing transcripts. In contrast, c-fos ARE-directed mRNA degradation proceeds through a different pathway not affected by these kinases.