For haemagglutination (HA), Immunoelectroosmophoresis (IEOP) and closed hexagon immundiffusion (CHI) experiments, the following antigen preparations have been employed : a) Crude suckling mouse brain (SMB) preparations: a 20 per cent infected suspension in APBS which has been centrifuged for 15 minutes at 10,000 rpm. b) Sucrose-aceton (SA) extracted infected mouse brains: