Helenalin is a sesquiterpene lactone first isolated from various species of Arnica. It has anti-inflammatory effects, most notably by inhibiting gene expression mediated by NF-κB at doses from 1 to 20 ?M. Like other sesquiterpene lactones, helenalin can modify sulfhydryl groups of cysteine residues and it alkylates these groups on the p65 subunit of NF-κB. It also suppresses the proliferation of cancer cells through multiple mechanisms, including the prevention of signaling through NF-κB. Helenalin has antibacterial and anti-protozoal activities.