Racing Victoria expects to know later this week whether Aidan and Joseph O'Brien's international spring contingent have been exposed to contaminated feed.

The trainers scratched their runners in Ireland and France over the weekend due to alleged contamination concerns.

RV acted immediately when they became aware of the issue and on Monday conducted out of competition testing on the 10 horses the trainers have in quarantine at Werribee.

The horses arrived on an international flight on Friday night.

"Reports from both trainers and international racing authorities are that feed used by both stables has been found in Europe to be contaminated with the prohibited substance Zilpaterol," RV said in a statement.

"RV stewards will continue to work with the O'Brien stables regarding the situation and will make a further comment once the result of the out of competition tests are known and the stables have been notified."

The test results are expected to take up to a week to be returned.

Zilpaterol is classified as a Beta-agonist, a medication which relaxes a horse's airway muscles and makes it easier for them to breathe.

They are allowed for therapeutic use but are a banned substance on race day.

As Beta-agonists are not classified as an anabolic steroid under the Australian Rules of Racing, they do not attract a mandatory stand down period.