USPS Homepage

USPS Packaging Instruction 6C

Category B Infectious Substances

Infectious substance means a material known or reasonably expected to contain a pathogen. A pathogen is a microorganism that can cause disease in humans or animals. Examples of pathogens include bacteria, viruses, fungi, and other infectious agents. An infectious substance must be assigned to one of the following two packaging categories:

Proper Shipping Name and ID Number


Required Packaging

  • Must be triple-packaged, meeting the packaging requirements in 49 CFR 173.199. Such materials must be properly packaged to withstand shocks, pressure changes, and other conditions related to ordinary handling in transit, and surrounded by absorbent material sufficient to protect the primary receptacle and to absorb the total amount of liquid should the primary receptacle leak or break.
Primary Receptacle (Container)
Cushioning and Absorbent Material
Secondary Container
Outer Shipping Container

As required by 49 CFR 173.199:

International Mail

Note: Countries distinguish between infectious and noninfectious (nonregulated) biological substances and may prohibit one or the other or both. See “Prohibitions” in the Individual Country Listings.