UFC 232 has been moved Los Angeles five days before it was due to take place Las Vegas over a drugs test issue with Jon Jones.
Jones was stripped of his light-heavyweight title after testing positive for a banned substance at UFC 214 in July 2017.
The American was due to fight Alexander Gustafsson in Nevada following a 15-month ban.
However, the United States Anti-Doping Agency says remnants of a banned substance have remained in Jones' system since the positive test.
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".
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".