South Korea coach Guus Hiddink has knocked down rumours claiming his team's remarkable stamina during this World Cup is down to doping.
Journalists at a Korea training session ahead of Saturday's quarter-final clash with Spain put the claims to Hiddink, though there seemed to be little substance to the rumours.
"I have never heard that, but if you accuse you have to prove," he snapped.
Hiddink added: "You can't improve players' speed with medication."
South Korea have emerged as the fittest team in the tournament and their constant work rate would deflate most other squads.
But Hiddink, himself soaked with sweat after another tough training session, insists the squad's stamina is simply down to hard work.
Korea's games v European opposition in 2002
He also suggested he may make changes to the team that beat Italy in order to introduce some fresh legs against Spain.
"We work very hard," Hiddink said.
"Everyone who is accusing us must come up with proof. They are more than welcome.
"But I don't want to react much on this because the boys have worked very hard."
Hiddink steered Holland to the World Cup semi-finals in 1998 and is hoping to emulate that feat by guiding South Korea to victory over Spain in Gwangju.