Kenyan Lawrence Cherono and American Randolph Ross have been suspended for doping offences, knocking them out of World Championships in Oregon, the Athletics Integrity Unit (AIU) said on Saturday.
Both men were due to compete on Sunday -- Cherono in the men's marathon and Ross in the 400m heats.
The AIU said in a statement that Cherono had tested positive for trimetazidine, a prohibited substance, after an out-of-competition test on May 23.
It said Ross was banned for allegedly "tampering with the doping control process".
Cherono, a 33-year-old, has won the Boston and Chicago marathons and was fourth in the Olympics in Tokyo last year.
The AIU said it was notified that the sample was positive by the World Anti-Doping Agency (WADA) laboratory in Lausanne on July 13, by which time Cherono was travelling to Eugene.
The AIU said the runner was told of the result when he arrived on Thursday and that he appealed, in vain, on Friday.
The Monaco-based AIU said it had complained to the laboratory that the positive result was reported 47 days after the test, even though WADA rules say labs have 20 days to deliver results.
The delay, said the AIU, "denied the opportunity for another Kenyan athlete to take the place of Mr Cherono in the marathon."
The 21-year-old Ross was part of the American gold-medal winning 400m relay squad in Tokyo, running in the heat but not the final.
The AIU said the allegation against Ross "arises out of the athlete's conduct during the course of an investigation into a potential whereabouts violation" following a missed test on June 18.