Tyler Hamilton told "60 Minutes" he used performance-enhancing drugs with Armstrong to cheat in cycling races, including the Tour de France.
He said Armstrong took a blood booster called EPO in the 1999 tour and before the race in 2000 and 2001. armstrong won the race every year from 1999 to 2005.
"I saw (EPO) in his refrigerator. ... I saw him inject it more than one time, like we all did. Like I did, many, many times," Hamilton said.
Shortly after Hamilton's comments aired, Armstrong refuted the claims and tweeted "20+ year career. 500 drug controls worldwide, in and out of competition. Never a failed test. I rest my case."
Armstrong's camp was quick to undercut the credibility of the accuser.
"Hamilton is actively seeking to make money by writing a book, and now he has completely changed the story he has always told before so that he could get himself on '60 Minutes' and increase his chances with publishers," Armstrong attorney Mark Fabiani said.
It will be up to the members of the grand jury to decide whether they believe the Hamilton who steadfastly denied doping for years - at times concocting elaborate excuses for his positive tests - or the one who testified under oath about the Armstrong case.
The Associated Press contributed to this story.