Behaviour in pigeons of an attenuated strain of Pigeon herpesvirus 1 (PHV1) was studied. After the first infection, it multiplied in the animal to the same extent as the natural strain it was derived from but only provoked mild and irregular symptoms. It persisted in the animal after infection. Primary infection of the pigeon with a pathogenic strain of PHV1 prevented recurrence of the disease after re-infection but did not significantly decrease viral dissemination and did not prevent spontaneous re-excretion of the virus.