The C-28 methyl ester of the oleane triterpenoid 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO-Me) induces apoptosis of human cancer cells by disrupting redox balance and is in clinical trials. CDDO-Me contains α,β-unsaturated carbonyl groups that form reversible adducts with thiol nucleophiles. The present studies show that CDDO-Me blocks interleukin-6 (IL-6)–induced and constitutive activation of the Janus-activated kinase 1 (JAK1) in cells. In support of a direct mechanism, CDDO-Me forms adducts with JAK1 at Cys¹⁰⁷⁷ in the kinase domain and inhibits JAK1 activity. In concert with these results, CDDO-Me blocked IL-6–induced and constitutive activation of signal transducer and activator of transcription 3 (STAT3). Moreover, we show that CDDO-Me (a) binds directly to STAT3 by a mechanism dependent on the alkylation of Cys²⁵⁹ and (b) inhibits the formation of STAT3 dimers. These findings indicate that CDDO-Me inhibits activation of the JAK1→STAT3 pathway by forming adducts with both JAK1 and STAT3. [Cancer Res 2008;68(8):2920–6]