A virus isolated from dead owls displaying a syndrom called hepato-splenitis infectiosa strigum (HSIS) killed chicken-embryos following inoculation by various routes. Miliary necrotic foci and type A nuclear inclusion bodies were observed on CAM and in embryonic livers and spleens. In chicken-embryo fibroblast tissue cultures syncytia, giant cells and type A nuclear inclusion bodies appeared. These features, as well as biochemical and biophysical properties described in another paper, allowed classification of HSIS virus as an herpesvirus. No serologic crossreaction was observed between HSIS virus on one hand, and infectious laryngotracheitis virus, Marek Virus, and 5 non-avian herpesviruses on the other hand. HSIS virus is therfore considered as a new avian herpesvirus.Herpesvirus strigis is proposed as an appropriate name, since owls of the orderstrigiformes are its natural host.