Variants of tick-borne encephalitis virus (TBA) isolated by inoculation of pig embryo kidney cell cultures (SPEV) with infectious DNA from chronically infected HEp-2-Soph cells produced cytopathic effect in SPEV cell cultures. In suckling mice inoculated intracerebrally they induced inapparent infection. Titers of TBE virus variants in SPEV cell cultures treated with DEAE-dextran increased considerably. As a result of filtration a variant was obtained replicating in SPEV cell cultures without any CPE. The cytopathic effect developed only when SPEV cells were treated with DEAE dextran. This variant is assumed to consist of two subpopulations 1 of which produces the cytopathic effect in DEAE-dextran-untreated SPEV cells while the other needs DEAE-dextran treatment of cells for its CPE manifestation.