UFC 232 has been moved to Los Angeles after a Jon Jones drug-test abnormality - five days before his fight against Alexander Gustafsson was due to take place in Las Vegas.
The United States Anti-Doping Agency says remnants of a banned substance have remained in Jones' system since a positive test on 29 July 2017 that led to a 15-month ban for the American.
The UFC says Jones, 31, tested positive for a "very, very small amount" of the metabolite on 9 December but added that it is "not a violation".
"This is the residual effect from the July 2017 test. This is not another positive test," said Jeff Novitzky, vice-president of athlete health and performance for UFC.
UFC president Dana White said the Nevada State Athletic Commission - which licenses the fight - does not have time to investigate the findings before Saturday's (29 December) promotion.
He added that the commission in California "has already dealt with Jon Jones and have all the facts, they have everything".
Jones says he has "willingly submitted" to every United States Anti-Doping Agency (Usasda) test in the lead-up to his light-heavyweight title fight against 31-year-old Gustafsson of Sweden.
"Usada has confirmed what I've been saying all along, that I'm a clean athlete," he posted on social media.
Novitzky said the amount of the long-term metabolite in Jones' system was like a "a grain of salt split into 50 million pieces".
White says tickets for Los Angeles will go on sale on Wednesday, 26 December. He has offered refunds to fans who had tickets for Las Vegas who now cannot attend.
Jones apologised to fans on his Instagram account, saying he "realises there's a lot of people who have lost out".
He added: "The situation is out of my control. I've been feeling the frustration of the fans, I'll do what I can to make it right. This is not a mistake I made."