Former UFC champion Conor McGregor's return to action helped trigger a split between the league and the U.S. Anti-Doping Agency, which has been testing the league's mixed-martial arts fighters for the past eight years.

McGregor announced on social media this week that he is returning to the USADA testing pool — a step the 35-year-old fighter must take before returning to the octagon for the first time since suffering a leg injury in July 2021. McGregor was expected to fight Michael Chandler in December, though the timing of their meeting has been in flux because of the testing rules.

USADA requires any athlete who has been out of action to submit to six months' of testing and provide at least two negative tests before returning.

In a statement Wednesday, USADA CEO Travis Tygart said the agency had been “clear and firm” about those requirements applying to McGregor, who during the summer went on social media and said that because of the testing rules “I feel like I'm being kept from my livelihood, and I've been feeling this for years.”

“Unfortunately, we do not currently know whether the UFC will ultimately honor the six-month or longer requirement because, as of January 1, 2024, USADA will no longer be involved with the UFC Anti-Doping Program,” Tygart said. “Despite a positive and productive meeting about a contract renewal in May 2023, the UFC did an about-face and informed USADA on October 9 that it was going in a different direction.”

The Associated Press sent an email to a UFC spokesman that was not immediately returned.

The bulk of USADA's work involves Olympic sports. When it partnered with UFC in 2015, it marked the agency's first deal with a U.