Some properties of togavirus hemagglutinin studied with the aid of kaolin-adsorbed virus.

Acidification (at pH 5.75) of Semliki Forest virus and West Nile virus suspensions completely eliminated their hemagglutinating activity within several minutes, but did not affect their infectivity or change their ability to absorb homologous hemagglutination-inhibition antibodies. In order to assay antibody absorption it was necessary to remove all of the immune complex from the reaction mixture, because the immune complex inhibited additional hemagglutinin. Removal of the immune complex can be accomplished by the use of kaolin-absorbed virus. This procedure is simple and dependable and has been carried out with viruses from several groups--Toga-, Myxo-, Paramyxo- and Picornaviridae.