---
type: restriction
id: usps-category-b-infectious-substances-un3373
asset: shipping-restrictions-by-class
title: "Category B infectious substances (UN3373) — Postal shipping restrictions by item category"
sources:
  - https://pe.usps.com/text/pub52/pub52c3_025.htm
source_quote: "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."
generated: true
harvested: 2026-08-04
stale_after: 2027-08-04
item_category: "Category B infectious substances (UN3373)"
carrier: "USPS"
restriction_type: "restricted"
scope: "domestic mail and limited international mail"
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"
verified: false
unverified_fields: conditions, item_category
---
**Item category:** Category B infectious substances (UN3373)

**Carrier:** USPS

**Restriction type:** restricted

**Scope:** domestic mail and limited international mail

**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

> 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>