Stewards have discovered syringes in a trainer's pocket and blood on the neck of a horse due to race at Mornington on Monday.
Members of Racing Victoria's compliance assurance team swooped on the Mornington stables of trainer Nathan McPherson as part of a campaign targeting the treatment of horses within 24 hours of them racing.
Acting chief steward Brett Wright said members of the team found a number of syringes on McPherson and others around his stables.
They also found what appeared to be blood and a mark on the neck of Testaguy who was due to race at Mornington on Monday.
At a subsequent inquiry, McPherson told the stewards he had been treating other horses in his stables with the contents of the syringes and had not treated Testaguy.
Stewards ordered the horse's scratching from its Mornington engagement.
Raids by Racing Victoria stewards in recent months have resulted in several high-profile trainers being charged, including Robert Smerdon and Sydney's Con Karakatsanis.
Caulfield trainer Tony Vasil is also under investigation after a raid on his stables last Friday.
No charges have yet been laid against McPherson and stewards have adjourned their inquiry into the matter until a date to be fixed.
Submissions on penalty in the Karakatsanis case will be heard on Thursday.