South African mountain bike rider David George, who rode with Armstrong's United States Postal Service team from 1999 to 2000, tested positive in an out-of-competition test on August 29, 2012, the South African Institute for Drug-Free Sport has announced.
SAIDS chief executive Khalid Galant said: "The blood test showed suspicious activity with regard to possible manipulation of the blood profile and a subsequent urine test came back positive for the banned EPO drug.
"His biological passport, which analyses the athletes blood profile indicated suspicious activity and that triggered a targeted test for EPO. EPO testing gives us a window of between six and 12 hours for testing because that's how long it will show up in a test."
Cycling South Africa president William Newman added: "Cycling South Africa respects the independence of the SAIDS process and will respect the outcome.
"Cycling SA further reiterates its zero-tolerance to doping in sport and confirms that there is no evidence of this being an endemic problem in the sport in South Africa."
Last month, the International Cycling Union (UCI) ratified the sanctions recommended by the United States Anti-doping Agency, who concluded Armstrong and his US Postal team ran "the most sophisticated, professionalised and successful doping programme that sport has ever seen".
All Armstrong's results from August 1, 1998 were expunged from the record books, including his seven consecutive Tour de France "wins" from 1999 to 2005, and the 41-year-old was banned for life.
Armstrong did not cooperate with the USADA investigation and has always denied wrongdoing.
In response to George's positive test, Nedbank suspended its sponsorship of his team.
"Today, Nedbank announced the immediate suspension of its sponsored professional cycling team, Team 360Life," a statement read.
"The suspension follows an adverse analytical finding in one of the samples taken during a random out of competition test of team rider David George. The suspension of the team will continue until further notice.
"Nedbank has a zero tolerance towards the use of any banned substances or performance enhancing drugs and does not condone or support such use in any sport.
"Nedbank will support the South African Institute for Drug-Free Sport, Cycling South Africa and Union Cycliste International in promoting clean conduct in the cycling fraternity."