Comparison of oligopeptide maps of several viruses from the tick-borne encephalitis complex revealed considerable similarity between them manifested in the presence of a common oligonucleotide framework. At the same time there are differences between viruses, each of them having its own oligopeptides lacking in other viruses. The viruses under comparison form groups: tick-borne encephalitis viruses proper, Langat and Powassan viruses, Negishi virus, louping-ill virus. Within the group of tick-born encephalitis viruses, neurovirulent strains differ from those with lower neurovirulence. Other variants of the method of oligopeptide mapping should be used to determine the significance of these differences.