Murine gammaherpesvirus 68 (MHV-68) -infected mouse is an animal model of gammaherpesvirus infection in man and domestic animals. Murine gammaherpesvirus 4556 (MHV-4556), isolated from Apodemus flavicollis ticks has been considered a close relative of MHV-68 but different in some features of infection in vitro and in vivo. Previous comparison of MHV-4556 with MHV-68 has revealed their diversity in immune evasion protein MK3. In this study, HindIII and EcoRI restriction profiles of the MHV-4556 genome disclosed absence of the deletion that has been identified previously at the left end of genomes of murine gammaherpesvirus 76 (MHV-76) and murine gammaherpesvirus Šumava (MHV-Šumava). A 22, 565 bp portion of MHV-4556 genome sequence was sequenced, analyzed and compared with that of MHV-68. Nucleotide sequences of 21 genes of MHV-4556 and deduced amino acid sequences revealed their identity to those of MHV-68 except for differences in 15 nucleotides and 8 amino acids in 5 genes and their proteins, respectively. Due to these differences, immune evasion protein M4 and structural proteins encoded by ORF8 (gB), ORF11 (p43), ORF26 and ORF52, respectively, are predicted to have a reduced hydrophilicity and surface exposure compard with their MHV-68 counterparts. These differences obviously contribute to some different pathogenetical features of these viruses and could explain the weaker immunogenicity of MHV-4556 in comparison with MHV-68.murine gammaherpesvirus 4556; restriction analysis; partial genome sequence.