It has previously been shown that infectious bursal disease virus is a naked icosahedral particle with a diameter of about 60 nm and a genome consisting of two segments of double-stranded RNA (Müller et al., J. Virol. 31:584-589, 1979). One of the two major structural polypeptides (molecular weight, 40,000) of this virus could not be found in lysates of infected cells; it is derived from a precursor polypeptide demonstrable inside the cells in relatively large quantities and seems to be processed during virus assembly or later. The precursor molecule is regularly present in the infectious virus particle (buoyant density, 1.33 g/ml) in minor proportions, but it represents an outstanding structural element of incomplete noninfectious particles ("top components"; buoyant density, 1.29 g/ml) which contain viral RNA. This type of incomplete particles is mainly produced by chicken embryo fibroblasts in contrast to lymphoid cells from the bursa of Fabricius. Precursor-product relationships also seem to exist in the biosynthesis of the other viral polypeptides. In contrast to some other viruses with a segmented double-stranded RNA genome, none of the structural proteins of infectious bursal disease virus is appreciably glycosylated.