Former double sprint world champion Tyson Gay is pulling out of next month's championships in Moscow following a failed drug test.
The American athlete said Sunday that the issue centred on a May out-of-competition sample and that he could not identify the substance.
A second sample from the same period has yet to be tested.
Gay, who had this year's fastest 100 metre sprint time of 9.75 seconds, holds the joint title of second fastest man in the world.
U.S. track and field officials said they were disappointed about the news ahead of a major event.
Jamaican sprinter Asafa Powell has also tested positive for a banned stimulant.
His agent told Reuters Sunday that Powell's team is now investigating how the substance got into his system during last month's national championships.
Jamaican compatriot and Olympic medallist Sherone Simpson admitted she had tested positive during the event as well.