Core proteins VP7 and VP3 have been localized in bluetongue virus (BTV) and BTV-infected cells by immunoelectron microscopy. Gold-labelled monoclonal antibodies to VP7 gave intense labelling with purified BTV core particles and weaker labelling with both directly visualized viral particles, which require no purification, and purified virus particles. It is believed that VP7 is, in a small number of viruses, accessible from the outer surface. The intensity of labelling by anti-VP7 antibodies was markedly increased by treatment of the virus with methanol. Intracellularly, VP7 antibodies also reacted with virus-like particles which appeared to be leaving virus inclusion bodies (VIB), the presumed site of virus synthesis and assembly. These antibodies also reacted with virus-like particles which were bound to the cytoskeleton and did not appear to be virus cores because they also reacted with gold-labelled antibody to the outer coat protein VP2. VP3 was not detected immunologically in either virus or core particles nor in cytoskeleton-associated virus-like particles suggesting an inner core location. VP7 and to a lesser extent VP3 were localized within the matrix of VIB. Virus tubules, a major structure found in infected cells and known to contain the non-structural protein NS1, were found to react with antibodies to both VP3 and VP7.