Effective purification methods have been developed for virus particles, infectious subviral particles (ISVP), and virus cores of bluetongue virus (BTV) serotypes 1 and 4. The purified particles were analysed by indirect ELISA or PAGE using either silver staining, or fluorography of [35S]methionine-labelled preparations. No significant contamination with host cell proteins, or with the majority of BTV nonstructural proteins was detectable in any of the particle preparations. In addition to the two major outer capsid and five core proteins previously described, the purified virus particles of both serotypes were consistently found to contain small amounts of BTV protein NS2, previously regarded as exclusively nonstructural. This protein could be removed from the particle surface by treatment with a combination of chymotrypsin and sodium N-lauroyl sarcosinate, which also resulted in the cleavage of the larger of the two major outer capsid components (protein VP2). Two of the cleavage products of VP2 and the whole of the other major outer capsid component (protein VP5) formed a modified outer capsid layer in the resultant ISVP. These subviral particles were as or more infectious than the intact virus particles but had lost haemagglutinating activity. The core-associated RNA polymerase remained inactive in ISVP.