Up to 31 athletes from six sports could be banned from competing at the Rio Games.
The announcement comes after the International Olympic Committee (IOC) retested 454 selected doping samples from the 2008 Games in Beijing.
The IOC said the retests were conducted using the very latest scientific analysis methods.
It also revealed it is awaiting the results of 250 retests from the 2012 Olympics in London.
IOC president Thomas Bach says all these measures are a powerful strike against the cheats they do not allow.
12 affected national Olympic associations would be informed in the coming days.
