A virus (HSIS) originating from dead owls was successfully cultivated in chicken embryo fibroblasts. Its replication was inhibited by IUDR. Tissue cultured virus proved sensitive to ether, chloroform, 0.5 per cent trypsin, and to pH levels of 4.0 or lower. Infectivity was rapidly destroyed at 56° C. Negatively stained naked virions of 100 nm average diameter were seen, and enveloped virions with 160–250 nm size. The capsid was built up of hollow cyclindrical capsomeres, arranged in equilateral triangles, carrying 5 capsomeres along each edge. Cubical symmetry and icosahedron structure yielded a total number of 162 capsomeres. All these biochemical and biophysical data lead to classification of HSIS virus into the genus herpesvirus. Biological properties described in a foregoing paper sustained such grouping, and indicated that the agent was a new avian herpesvirus for which the nameherpesvirus strigis was proposed.