The UFC announced dos Santos' removal from the event on Friday. The promotion will try to book a new opponent for Ngannou.
The United States Anti-Doping Agency notified the UFC on Friday that dos Santos (18-5) had been flagged for a potential violation stemming from an out-of-competition test administered Aug. 10. No additional information was provided, which is standard under the UFC's anti-doping program.
Manager Ana Guedes released a statement later Friday, saying dos Santos tested positive for hydrochlorothiazide, a diuretic banned by USADA because it can be used as a masking agent.
"We do not know where this substance came from," Guedes said. "Junior has had his blood and urine randomly tested many times and has never once tested positive for a banned substance.
"We requested testing of his B sample today. Most times, the B sample will show the identical results as the A sample. Therefore, we are already working on analyzing his nutritional supplements and medications, to determine whether anything might have been contaminated."
Guedes said dos Santos spoke to Ngannou and apologized.
"Junior thanks Francis for the sympathy, understanding and respect that transpired during their call, and very much looks forward to resolving this situation so that he and Francis can set up a new date to face one another," he said.
Ngannou (10-1), who trains out of the UFC's Performance Institute in Las Vegas, was quick to respond to the initial news on social media.
I'm very very angry!!!— Francis NGannou (@francis_ngannou) August 18, 2017
Almost done with training camp and they just announced that my opponent is out for #ufc215
Born in Cameroon, Ngannou is on the verge of title contention. He is 5-0 in the UFC and was favored to defeat dos Santos.
Dos Santos, 33, is coming off a first-round knockout loss to Stipe Miocic in a UFC title fight in May.