The capacity of 3 members of the tick-borne encephalitis virus complex (Langat Tp-21, Sophyin, Elantsev) differing in their biological properties to induce chronic infection in primary cultures of suckling Syrian hamster brain cells (SHB) was studied. Three types of the infectious process were observed in these cells. Langat Tp-21 virus showed cytoproliferative activity in chronically infected SHB cell cultures; formation of cell colonies and alternation of phases of destruction and repopulation were observed with persisting Sophyin strain. The Elantsev strain in these cells produced infection with unestablished virus-cell equilibrium. The persisting viruses were shown to undergo changes in their biological properties consisting in the loss of the hemagglutinating activity and reduced pathogenicity for the susceptible animals.