Newcastle Knights player Jarrod Mullen has been provisionally suspended under the NRL's Anti-Doping Policy.
A test on November 29 returned a positive A-sample for the anabolic steroid Drostanolone, which is prohibited by the World Anti-Doping Agency (WADA).
Mullen, who is banned from playing while the provisional suspension is in place, now has the opportunity to have his B-sample analysed.
"The club is obviously extremely disappointed," Knights CEO Matt Gidley said.
"The club has a strict governance program, the players are regularly educated and fully aware of the consequences of going outside our governance guidelines.
"From here, there is a formal process we need to follow under the guidelines.
"We need to respect the process and in the interim the club will continue to monitor Jarrod's welfare."
Mullen, 29, has played 211 matches since making his Knights debut in 2005 and is an important member of the first team at the club, who finished bottom of the ladder last season.
The Knights' first match of the 2017 campaign is away to New Zealand Warriors on March 5.