Hemagglutinating antigens of Western equine encephalomyelitis (WEE) virus and 5 strains of tick-borne encephalitis (TBE) virus were charcaterized by their capacity to adsorb on inorganic (kaolin, bentonite, barium sulphate) and organic (chick embryo fibroblasts) adsorbents. The adsorption capacity of TBE virus was more marked than that of WEE. The TBE virus strains 4072 and 1524 avid in serological tests adsorbed better on inorganic adsorbents than nonavid strains No. 80 and 1509. In contrast, nonavid strains interacted better with biologic adsorbents. The adsorption capacity of tissue culture antigens was superior to that of brain hemagglutinating antigens. By means of adsorption on kaolin and chick embryo fibroblasts hemagglutinating and nonhemagglutinating infections particles of WEE and TBE viruses which also differed in some biological properties were separated.