The properties of virions produced in pig embryo kidney (PEK) cells inoculated with tick-borne encephalitis (TBE) virus strain EK-328 which had been passaged in white mice and its variant obtained by passages of TBE in H. plumbeum ticks (clone 718/574 H. pl.17) were found to be different. The clone 718/574 H. pl17 virus particles had no hemagglutinating or precipitating activities, greater sedimentation and density heterogeneity in sucrose density gradient centrifugation, and differences in movements to electrodes in electrophoresis and immune electrophoresis. In mixed infection in PEK cultures with EK-328 strain and clone 718/574 H. pl17, the infective dose-dependent interference was observed which affected the infectious virus titre and the size of cathode antigen precipitate.