U.S. Postal Code C203, Section 9.0, states:

Radioactive materials are prohibited in international mail and domestic mail if required to bear the DOT Radioactive White-I, Radioactive Yellow-II, or Radioactive Yellow-III label (49 CFR 172.436, 172.438, or 172.440, respectively) or if it contains quantities of radioactive material in excess of those authorized in Publication 52, Acceptance of Hazardous, Restricted, or Perishable Matter. Radioactive materials are prohibited in domestic mail via air transportation. For international mail, the standards in IMM 135 apply.