Kenyans Joyce Zakary and Koki Manunga have become the first athletes to fail drugs tests at the World Championships in Beijing.
The International Association of Athletics Federations announced that the pair had accepted provisional suspensions after testing positive before competition got under way.
An IAAF statement said: "These targeted tests were conducted by the IAAF at the athlete hotels during the pre-competition phase. The IAAF will not discuss the details of the cases as they progress through the results management process."
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Zakary, whose positive test was for a sample provided last Friday, set a new Kenyan 400 metres record of 50.71 seconds in the heats, but the 29-year-old did not start Tuesday's semi-final.
Manunga's positive sample was from the day before. The 21-year-old finished second last in her 400m hurdles heat on Sunday.
The positive tests will take the shine of what has been an impressive championships for Kenya so far. They topped the medal table after the opening four days with nine medals, including four golds.
Athletics in Kenya has come under close scrutiny following a spate of positive tests from their athletes over the past few years.
Doping allegations overshadowed the build-up to the championships, with damaging claims, fiercely denied by the IAAF, that it turned a blind eye to suspicious blood test results from hundreds of athletes.
In the lead-up to the global showpiece in China, the world governing body announced that 28 athletes had been suspended over historic doping offences.
The fight against doping is set to be the key issue which will define incoming IAAF president Lord Coe's time in charge of the organisation.