Alex Rodriguez and Ryan Braun are among the players being interviewed as part of Major League Baseball's inquiry into Biogenesis, a closed Florida anti-aging clinic accused of distributing banned performance-enhancing drugs.
"Repeated leaks threaten to harm the integrity" of the drug agreement, union executive director Michael Weiner said in a statement. He added that the leaks "call into question the required level of confidentiality needed to operate a successful prevention program."
Braun is among several players who refused to answer questions about their dealings with Biogenesis, which was first reported Wednesday by ESPN and the Daily News of New York. Braun was interviewed last month and Rodriguez is scheduled to be interviewed Friday.
"The players want a clean game and they demand a testing program that is not only the toughest in professional sports, but one that guarantees each player due process rights accompanied by strict confidentiality provisions," Weiner said.
The union said it has no information about the source of the leaks or any indication that MLB is the source.
MLB has spent most of the year investigating about 20 players for their links to Biogenesis, including Rodriguez and Braun, both former MVPs. Miami New Times reported in January the clinic had distributed banned drugs to major leaguers.
Braun and Rodriguez have said they didn't do anything that merits discipline.
MLB aims to complete the player interviews in mid-July, but is not sure whether it will meet that schedule. Management then will have to decide on possible discipline, which is likely to be challenged in grievances before an arbitrator.
"It would be unfortunate if anyone prejudged the results of the investigation based on unsubstantiated leaks that are a clear violation of the JDA," Weiner said, referring to baseball's joint drug agreement.
Rodriguez has said he used performance-enhancing drugs while with Texas from 2001-03 but has denied using them since.
Braun's 50-game suspension was overturned last year by an arbitrator who ruled the Milwaukee star's urine sample was mishandled by the drug collector.