THE AFL Anti-Doping Tribunal has spent the past two days hearing submissions from Essendon players and the Australian Sports Anti-Doping Authority on the admissibility of certain evidence relating to the case.

David Grace QC launched the defence of the Essendon players on Tuesday before both sides called on medical specialists to give evidence on Wednesday.

In December, ASADA lost a legal battle to have two of its key witnesses, biochemist Shane Charter and compound pharmacist Nima Alavi, testify at the hearing.

Charter and Alavi have given evidence to ASADA but refused to sign sworn affidavits backing the authority's allegations that the players were administered the banned drug thymosin beta-4 as part of Essendon's 2012 supplements program.

Lawyers for the players are arguing the players were given a legal version of thymosin - thymosin alpha-1 or thymomodulin.

The hearing is being held in private at the Victorian County Court after a ruling by Tribunal chairman David Jones on December 8 last year.

The hearing will continue on Tuesday.