# Category B infectious substances (UN3373) — Postal shipping restrictions by item category

For Category B infectious substances (UN3373), carrier is USPS; restriction type is restricted; scope is domestic mail and limited international mail; conditions is Mailable only when intended for medical, veterinary, research, or public health lab use; must be properly prepared; domestic mailing requires Priority Mail or Priority Mail Express; international requires written approval from director Product Classification, recorded from its source on 2026-08-04.

- **Item category:** Category B infectious substances (UN3373) _(unconfirmed — we could not tie this to the quote below)_
- **Carrier:** USPS _(our reading, not quoted from the source)_
- **Restriction type:** restricted _(our reading, not quoted from the source)_
- **Scope:** domestic mail and limited international mail _(our reading, not quoted from the source)_
- **Conditions:** Mailable only when intended for medical, veterinary, research, or public health lab use; must be properly prepared; domestic mailing requires Priority Mail or Priority Mail Express; international requires written approval from director Product Classification _(unconfirmed — we could not tie this to the quote below)_

## What the source says

> Domestic Mail: Infectious substances are mailable only when intended for medical or veterinary use, research, or laboratory certification related to public health, and must be properly prepared to withstand shocks, pressure changes, and other conditions incident to ordinary handling in transit.

## Source

- https://pe.usps.com/text/pub52/pub52c3_025.htm

Last verified: 2026-08-04. Review by: 2027-08-04.
Part of [Postal shipping restrictions by item category](https://referencesource.org/shipping-restrictions-by-class/).