Electron microscopic studies of morphological lesions in pig embryo kidney cells (PEK) infected with tick-borne encephalitis (TBE) virus as well as morphology and features of TBE morphogenesis after treatment with actinomycin D, cycloheximide and hypertonic NaC1 concentrations in the medium were carried out. Most marked morphological lesions were observed in the cells after combined effect of high NaC1 concentrations in the medium and inhibitors of protein synthesis. After all kinds of treatment, smooth-contour membrane structures were observed in TBE-infected cells. Their number increased considerably with increasing ionic strength of the medium and subsequent return to normal accompanied by treatment with actinomycin D and cycloheximide. No "budding" particles were found in any case and after any treatment. Features of TBE virus morphogenesis are discussed.