Super-featherweight world champion Alycia Baumgardner has returned an “adverse analytical finding” following a drug test, Matchroom Boxing has said.
The test was taken before the American, 29, defeated Greece’s Christina Linardatou in Detroit last month.
In a statement, Matchroom said it is “committed to promoting a clean and fair sport for all athletes”.
Baumgardner took her record to 15 wins from 16 fights when she beat Linardatou on points by unanimous decision.
Matchroom said it would assist Baumgardner and her team during this time, “affording her the appropriate due process”.
The statement added: “Matchroom was first informed of this result on 12 August.
“The applicable regulatory authorities are also aware and we defer to such bodies for further action and guidance as a full investigation is conducted.
“As we wait to receive further information, Matchroom will not be making any further comment.”
Ohio-born Baumgardner holds all the titles – WBA, WBC, IBF, WBO and The Ring – in the super-featherweight category.
She hinted in her post-fight interview with Linardatou she would be willing to step up a weight division – from super-featherweight at 9st 4lb (59kg) to lightweight at 9st 9lb (61.4kg) – to take on the Republic of Ireland’s Katie Taylor.
It is the second anti-doping controversy to hit boxing in two weeks after Dillian Whyte’s “adverse finding” led to the cancellation of his bout against Anthony Joshua last Saturday.