Final USDA Rule: Traceability for horses moving interstate – 2013: 

Accepted methods of official identification are one of the following:
1. A microchip (electronic identification) that complies with ISO 11784/11785. (15 digits long)
2. A microchip (electronic identification) that is Non-ISO is accepted if it was installed prior to March 11, 2014.
3. A written description sufficient to identify the individual equine including, but not limited to, name, age, breed, color, gender, distinctive markings, and unique and permanent forms of identification when present (e.g., brands, tattoos, scars, cowlicks, blemishes or biometric measurements). When the identity of the equine is in question at the receiving destination, the State or Tribal animal health official in the State or Tribe of destination or APHIS representative may determine if the description provided is sufficient.
4. Digital photographs sufficient to identify the individual equine.

Note:  Microchip ID Equine is an approved manufacturer of the United States 840 microchip.  The USDA issues to us an official list of unique numbers all beginning with the USA official prefix – 840 .  We manufacture the microchips under strict quality control to comply with Choice 1. above.

And…Check out the USA 840 Microchip. The ISO chip vetted and approved for horses by the USDA.