Kjærgaard said he started doping on his own initiative in 1998 when with the Danish Chicky World team, going on to meet with Belgian doctor George Mouton for "advice" to "avoid the risk of being caught".
The Norwegian then joined the US Postal team and raced alongside Armstrong in the 2000 and 2001 Tours. He said doping at the time was endemic and the team arranged all.
"Everything was organized by the team," Kjærgaard told a press conference.
"I can personally assume that there were a certain number of others involved in doping," he added.
Kjærgaard's comments follow the fall from grace of Armstrong, who has been stripped of his seven Tour de France titles.
The International Cycling Union (UCI) on Monday gave its backing to a damning US Anti-Doping Agency (USADA) dossier that placed the Texan at the heart of the biggest doping programme in sport, erasing his record back to August 1st 1998.
Eleven other former members of US Postal have previously told their stories of doping with the professional cycling team, notably Tyler Hamilton.