PARIS — It could be truth or dare time on Lance Armstrong's comeback trail.

The French anti-doping authority has thrown down a challenge to the seven-time Tour de France champion, proposing he agree to retesting of his 1999 urine samples to see whether a French newspaper was right when it reported they contained traces of EPO, a banned blood-boosting hormone that enhances endurance.

A positive test from the samples could not lead to a ban that would thwart the 37-year-old star's return to cycling after three years in retirement.

Too much time has passed for disciplinary measures to be taken and only part of Armstrong's samples were kept. Even so, the proposal renews debate about one of the most contested questions surrounding Armstrong: whether he was clean when he won. He has always insisted that he was, and his new team, Astana, is hiring a drug-testing expert for Armstrong's comeback to try to silence doubters.

In a statement, the agency headed by Pierre Bodry proposed the rider "prove his good faith" by agreeing to retesting of his samples from the 1999 Tour, the first in Armstrong's record string of seven wins. The agency said it was acting in the interests "of objectivity and of justice and to allow the cyclist Lance Armstrong to cut short the rumors concerning him, if they are unfounded."

Armstrong responded in a statement.

"Unfortunately, Mr. Bodry is new to these issues and his proposal is based on a fundamental failure to understand the facts," Armstrong said.

"In 2005, some research was conducted on urine samples left over from the 1998 and 1999 Tours de France. That research was the subject of an independent investigation, and the conclusions of the investigation were that the 1998 and 1999 Tour de France samples have not been maintained properly, have been compromised in many ways, and even three years ago could not be tested to provide any meaningful results. There is simply nothing that I can agree to that would provide any relevant evidence about 1999."

In drug testing, urine is divided into "A" and "B" samples, and both must show traces of a banned substance for the test to be declared positive.

Only remains from six "B" samples have been kept from Armstrong's 1999 Tour, the French agency said. So even if the "B" samples came back positive in new testing, there are no "A" samples left against which to compare results.

The samples are frozen in a drug testing laboratory in the suburbs of Paris. They've been a source of controversy since L'Equipe reported in 2005 that a new round of tests on the "B" samples found EPO, a blood-boosting hormone that enhances endurance.

Armstrong said then he was the victim of a "witch hunt." A Dutch lawyer appointed by cycling's governing body later cleared Armstrong. But Dick Pound, who then led the World Anti-Doping Agency, said the lawyer's findings were full of holes.

The French agency said sufficient urine remains to conduct EPO tests from at least five days of racing at the 1999 Tour. If Armstrong were to agree, tests could be carried out quickly at the lab in the presence of a representative for the rider, the agency said. Or, if Armstrong prefers, they could be done at an another WADA-accredited European lab outside France.

Quoted by L'Equipe, Pierre Bordry said he wanted to act as "a referee" between the newspaper and Armstrong. But Bordry seemed to already have an opinion, speaking to the newspaper of samples "which contain erythropoietin (EPO)."

"I want this comeback to take place in the best circumstances," L'Equipe quoted him as saying in its Wednesday edition. "If he agrees, we'll launch the operation."

He added: "This way, he will perhaps have the chance to affirm that he never cheated during his brilliant career."