Although cisplatin has been used for decades to treat human cancer, some toxic side effects and resistance are observed. It has been suggested that gold(III) complexes, containing metal centers isoelectronic and isostructural to cisplatin, are promising anticancer drugs. Gold(III) dithiocarbamate complexes were shown to exhibit in vitro cytotoxicity, comparable with and even greater than cisplatin; however, the involved mechanism of action remained unknown. Because we previously reported that copper(II) dithiocarbamates are potent proteasome inhibitors, we hypothesized that gold(III) dithiocarbamate complexes could suppress tumor growth via direct inhibition of the proteasome activity. Here, for the first time, we report that a synthetic gold(III) dithiocarbamate (compound 2) potently inhibits the activity of a purified rabbit 20S proteasome and 26S proteasome in intact highly metastatic MDA-MB-231 breast cancer cells, resulting in the accumulation of ubiquitinated proteins and the proteasome target protein p27 and induction of apoptosis. The compound 2–mediated proteasome inhibition and apoptosis induction were completely blocked by addition of a reducing agent DTT or N-acetyl-l-cysteine, showing that process of oxidation is required for proteasome inhibition by compound 2. Treatment of MDA-MB-231 breast tumor–bearing nude mice with compound 2 resulted in significant inhibition of tumor growth, associated with proteasome inhibition and massive apoptosis induction in vivo. Our findings reveal the proteasome as a primary target for gold(III) dithiocarbamates and support the idea for their potential use as anticancer therapeutics. (Cancer Res 2006; 66(21): 10478-86)