The World Anti-Doping Agency (Wada) is to appeal against a decision to clear 34 players associated with Australian Rules team Essendon of doping.
Wada director David Howman stated his agency will contest the decision to the Court of Arbitration for Sport (CAS).
Essendon chief Paul Little said Wada's actions came as "a surprise".
An Australian Football League tribunal had concluded it was not "comfortably satisfied" any player was administered a banned peptide substance.
In 2013, Little's Australian Football League (AFL) team were investigated by the Australian Sports Anti-Doping Authority (Asada) into supplements administered to players the previous season.
|Thymosin beta 4|
|The substance is not mentioned by name on Wada's banned list, but is a peptide which comes under Wada's S2 category of performance enhancing drugs.|
Essendon were one of two Australian Rules teams along with six top-flight rugby league clubs in the country under scrutiny following a report suggesting doping in sport in the country.
The club were initially fined Aus $2m (£1m) by the AFL and banned coach James Hird for 12 months following an interim report by Asada that highlighted management failings regarding the possibility that players could have doped.
In 2014, Asada then acted on 34 Essendon players - past and present - it believed were allegedly administered a banned peptide thymosin beta 4, which promotes muscle growth, by Essendon sports scientist Stephen Dank. The players were handed a provisional suspension pending an AFL tribunal.
That tribunal in March 2015 cleared the players, while Asada opted not to appeal against the verdict.
Dank was also cleared of 21 breaches of the league's drug code, but was found guilty of 10 charges of "trafficking, attempting to traffic and complicity in matters related to a range of prohibited substances". Dank will appeal against those charges.