Immunodiffusion between hyperimmune serum (HI) in the centre well and whole virus (146S), pentameric viral subunits (12S) and tissue culture supernatant (TC). Single lines of precipitation are observed between the serum and either the whole virus or the 12S subunits. Three lines of precipitation occur between tissue culture supernatant and the serum, representing whole virus, 12S subunits and the virus infection associated antigen (VIAA), with the VIAA line being furthest away from the antigen well.