It is illegal to bite your opponent in a professional fight.

The Law Explained

The Utah Athletic Commission has a highly detailed list of fouls for professional boxing and mixed martial arts, and biting is strictly at the top. While this seems obvious, sports commissions have had to codify the ban on cannibalistic combat ever since the infamous Mike Tyson ear-biting incident. Biting, spitting, or headbutting will get your license revoked immediately.