Gliotoxin-13C13is intended for use as an internal standard for the quantification of gliotoxin by GC- or LC-MS. Gliotoxin is an immunosuppressive mycotoxin produced by pathogenic strains ofAspergillusand other fungi with diverse biological activities.1,2,3,4,5,6,7,8It inhibits 20S proteasomal chymotrypsin activity (IC50= 10 μM), blocking the degradation of IκBα and preventing the activation of NF-κB.2,3Gliotoxin induces apoptosis in monocytes and dendritic cells and reduces phagocytosis by neutrophils.4,5It suppresses viral infection by Nipah and Hendra virus in HEK293T cells (IC50s = 149 and 579 nM, respectively).6Under reducing conditions, gliotoxin inhibits leukotriene A4hydrolase epoxide hydrolase activity, but not aminopeptidase activity, and leukotriene B4synthesis in neutrophils and monocytes.7