Kenya's only Olympic boxer was thrown out of the 2004 Games on Tuesday after becoming the first competitor to test positive in Athens for a banned drug.
"David Munyasia has been excluded from the Athens Olympic Games after testing positive for cathine, a prohibited stimulant," IOC spokeswoman Giselle Davies told reporters.
The substance derives from the leaf of the qat plant, commonly chewed as a recreational drug in east Africa.
But IOC medical director Patrick Schamasch said it was unclear how the 24-year-old bantamweight came to absorb it. "There was no explanation in defence," Schamasch said.
It was the first positive case among more than 200 dope tests conducted in Athens since late July among athletes arriving for the Games, which begin on Friday. Six others have been barred recently for failing pre-Olympic tests elsewhere.
Tested last Friday, Munyasia's Olympic accreditation had been withdrawn and the IOC had contacted the international boxing authorities to consider further measures, Davies said.
Athletes who fail a drugs test can expect to be banned for at least two years. The Athens Games is the first Olympics following the introduction of a global anti-doping code.
"It is never pleasant to announce a positive drugs test, but that we have found an athlete, not playing by the rules, is a confirmation of the IOC's determination to stamp out doping in sport," Davies said.
"YOU ARE GOING TO BE SURPRISED"
Munyasia, who had been training in Cuba, was the only boxer left in Kenya's 40-strong Olympic squad after flyweight medal hope Suleiman Bilali dropped out due to injury.
"I am travelling as an underdog but you are all going to be surprised," Munyasia was quoted as saying before heading for Athens. Kenyan officials could not be reach for comment.
The IOC is testing for stimulants such as those detected in Munyasia's system out of competition for the first time at the Athens Games. In previous Olympics, only steroid-use was probed before competition actually started.
"We are testing for every banned substance on the list for the duration of the Games," chairman of the IOC's medical commission Arne Ljungqvist told reporters. "We analyse for the full range of substances."
So far, 224 doping tests had been carried out of a total of 3,000 to be completed by the end of competition on August 29, Davies added -- 25 percent more tests than the Sydney Olympics.
Irish distance runner Cathal Lombard faces a two-year ban after he confirmed on Tuesday that he took a banned performance-enhancing drug during his pre-Olympic training. Lombard, 28, said he made a "grave error" and would not appeal against his failed test for erythropoietin (EPO), a blood-boosting substance which enhances endurance.
"I have caused a lot of hurt," said the lawyer, who had slashed 10 percent off his 10,000 metres time in the past year.