CD40, a member of the TNF receptor superfamily, is critical for productive immune responses. Macrophages constitutively express CD40 at low levels, which are enhanced by IFN-γ. IFN-γ-induced CD40 expression involves activation of STAT-1α as well as NF-κB activation through an autocrine response to IFN-γ-induced TNF-α production. Statins are 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors, which exert antiinflammatory effects independent of their cholesterol-lowering actions. Herein, we describe that simvastatin (SS) inhibits IFN-γ-induced CD40 expression via the suppression of STAT-1α expression. This results in diminished STAT-1α recruitment to the CD40 promoter upon IFN-γ treatment, in addition to reduced RNA Polymerase II recruitment and diminished levels of H3 and H4 histone acetylation. SS-mediated inhibition of STAT-1α occurs through suppression of constitutive STAT-1α mRNA and protein expression. The inhibitory effect of SS on CD40 and STAT-1α is dependent on HMG-CoA reductase activity, as the addition of mevalonate reverses the inhibitory effect. In addition, CD40 and/or STAT-1α expression is inhibited by GGTI-298 or Clostridium difficile Toxin A, a specific inhibitor of Rho family protein prenylation, indicating the involvement of small GTP-binding proteins in this process. Collectively, these data indicate that SS inhibits IFN-γ-induced CD40 expression by suppression of STAT-1α, and altering transcriptional events at the CD40 promoter. © Society for Leukocyte Biology.