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-α). Macrophages derived from these mice oversecrete TNF-α, by a mechanism that involves stabilization of TNF-α mRNA, and TTP can bind directly to the AU-rich element (ARE) in TNF-α mRNA (E. Carballo, WS Lai, and PJ Blackshear, Science 281: 1001–1005, 1998). We show here that TTP binding to the TNF-α 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-α ARE. In intact cells, TTP at low expression levels promoted a decrease in size of the TNF-α mRNA as well as a decrease in its amount; at higher expression levels, the shift to a smaller TNF-α 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-α mRNA. This CCCH protein is likely to be important in the deadenylation and degradation of TNF-α mRNA and perhaps other ARE-containing mRNAs, both in normal physiology and in certain pathological conditions.