Rodriguez is baseball's highest paid star
Major League Baseball has refused to confirm reports that Alex Rodriguez, the sport's biggest name, tested positive for anabolic steroids in 2003.
Sports Illustrated magazine claimed that New York Yankees star Rodriguez, 33, failed a drugs test while he was playing for the Texas Rangers.
MLB said its testing programme at the time was "intended to be non-disciplinary and anonymous".
Third baseman Rodriguez refused to comment on the allegations.
"You'll have to talk to the (players) union," he told a Sports Illustrated reporter in Miami.
So far, neither the head of the Major League Baseball Players Association, Donald Fehr, has made any public comment, nor have the Yankees.
Sports Illustrated claimed its allegations about Rodriguez are supported by four independent sources.
But MLB said it was "disturbed" by any suggestion that the results of its 2003 drugs tests had entered the public domain.
"We cannot make any comment on the accuracy of this report as it pertains to the player named," it said in a statement.
"Based on the results of the 2003 tests, MLB was able to institute a mandatory random testing programme with penalties in 2004."
MLB claimed it now has "the toughest [testing] programme in professional sports".
While MLB's drug policy has banned the use of steroids without a valid prescription since 1991, there were no penalties for a positive test in 2003.
That year's survey testing was conducted as part of an agreement with the players' union to determine if it was necessary to impose mandatory random drug-testing across the major leagues.
Sports Illustrated has alleged that Rodriguez, baseball's highest paid star, on 10-year-deal worth up to $305m, was one of 104 players to test positive.