Mycophenolic acid-beta-D-glucuronide, also known as MPAG, is the primary metabolite of the immunosuppressant mycophenolic acid and is monitored by clinical labs to ensure patients remain within the drug’s therapeutic range. This internal standard is suitable for quantification of mycophenolic acid glucuronide levels in whole blood or plasma by LC-MS/MS for therapeutic drug monitoring and other clinical or diagnostic applications.