The aim of this study was to investigate whether and how a non-chasing and a chasing companion dog influence sheep chasing in test dogs (20 Norwegian Elkhounds, grey), and whether this varies with sex or age. The test dogs’ predatory behaviour towards sheep was examined by observation of the dogs in a fenced enclosure with sheep in 5-minute tests together with, first, a non-chasing (Hamilton stoever, a hound breed) and 2-3 days later a chasing (Border collie) companion dog. Physical contact with sheep was prevented. Initially, the test dogs exhibited a higher chasing motivation towards sheep in tests with a chasing, as compared with a non-chasing, companion. During the entire test, 60% of the dogs attacked sheep when accompanied by the non-chaser. All dogs attacked sheep when the chasing companion was present, although in only 8 of 20 cases the companion dog chased simultaneously with the test dog. In the chasing-companion tests, the attack latency was shorter than in tests with a non-chasing companion. The attack severity was higher when attacking alone than when attacking together with the chasing companion. In these tests, the higher the attack severity, the shorter was the attack latency. A sequence analysis on chasing-companion tests showed that test dogs generally started with observing or showing interest in sheep, followed by attacks, which increased in severity. Taken together, our findings indicate that a companion dog showing intentions of predatory behaviour stimulates predatory chase in another dog, while a non-chasing companion has a limited influence on this. In tests certifying dogs for their refrainment of chasing sheep, well trained Border collies approaching sheep on command might be used to reveal the full predatory potential of the dog being tested.