Mice deficient in tristetraprolin (TTP), the prototype of a family of CCCH zinc finger proteins, develop an inflammatory syndrome mediated by excess tumor necrosis factor alpha (TNF-␣). Macrophages derived from these mice oversecrete TNF-␣, by a mechanism that involves stabilization of TNF-␣ mRNA, and TTP can bind directly to the AU-rich element (ARE) in TNF-␣ mRNA (E. Carballo, W. S. Lai, and P. J. Blackshear, Science 281:1001-1005, 1998). We show here that TTP binding to the TNF-␣ ARE is dependent upon the integrity of both zinc fingers, since mutation of a single cysteine residue in either zinc finger to arginine severely attenuated the binding of TTP to the TNF-␣ ARE. In intact cells, TTP at low expression levels promoted a decrease in size of the TNF-␣ mRNA as well as a decrease in its amount; at higher expression levels, the shift to a smaller TNF-␣ mRNA size persisted, while the accumulation of this smaller species increased. RNase H experiments indicated that the shift to a smaller size was due to TTP-promoted deadenylation of TNF-␣ mRNA. This CCCH protein is likely to be important in the deadenylation and degradation of TNF-␣ mRNA and perhaps other ARE-containing mRNAs, both in normal physiology and in certain pathological conditions.