In this paper, we compared the bioavailability of penicillamine selenotrisulfide (PenSSeSPen) with the authentic selenium-source compound, selenite. When PenSSeSPen was orally administered to selenium-deficient mice for 7 days, selenium contents of blood, liver and heart significantly increased as well as selenite. In addition, when PenSSeSPen-supplemented diet was fed to 3-week old mice for 7 weeks, selenium contents of selected organs were almost the same as those of selenite, and hepatic cellular glutathione peroxidase (GPx) activity was also observed. These facts evidently indicate that PenSSeSPen, a totally foreign selenium compound, serves as a selenium-source compound.