Biological properties and restriction enzyme patterns of the slowly replicating herpesviruses isolated from cattle affected with different diseases in North America and Europe were analysed. These virus isolates induced identical plaques that developed within 7 to 9 days in bovine foetal spleen cells and within 5 days in actively growing Georgia bovine kidney cells. These virus isolates were found to be antigenically related when tested in the indirect immunofluorescence test, and antigenic relationships with bovine herpesvirus 1 (BHV-1), BHV-2, BHV-3 or BHV-6 were not detected. The genomes of these strains were shown to have virtually identical cleavage sites when treated with restriction enzymes EcoRI, BamHI, SstII, SphI and HindIII. The resulting restriction enzyme patterns differed strikingly from those of BHV-1, BHV-2, BHV-3 and BHV-6. Because the herpesviruses tested become enveloped on the nuclear as well as on endoplasmic membranes, a process through which they induce cytoplasmic vesicles filled with enveloped viral particles, and because of the unique cytoplasmic inclusions that are induced, we classify them tentatively as bovine cytomegaloviruses.