Tick-borne encephalitis (TBE) virus grown in suspensions of chick embryo cells was precipitated by various concentrations of polyethylene glycol (PEG) 6000. Quantitative recovery was obtained at PEG concentrations of 5% and higher. As precipitation of contaminating non-viral protein increases with increasing PEG concentrations, best results with respect to purity and recovery were obtained at 5% PEG 6000. Analysis of virus concentrates by rate zonal centrifugation revealed two peaks--rapidly sedimenting haemagglutinin (RHA) associated with infectivity and slowly sedimenting haemagglutinin (SHA)--characteristic of Flaviviruses. Purification factors after PEG precipitation followed by rate zonal centrifugation in sucrose density gradients ranged from 50 to 200 for HA activity and from 40 to 50 for infectivity.