Bovine enterovirus-I (BEV-I) infection results in the production of a low amount of infective virus. A large number of non-infectious virus particles can be detected in BEV-I lysates by haemagglutination. Attempts to isolate DI particles that might be responsible for this effect failed. However, infected cells were shown to contain large amounts of 80S particles as well as lesser amounts of 160S, 130S, 45S, 14S and 5S particles. The proportion of these subviral particles detectable by density gradient sedimentation depended on the ionic strength of the gradient buffer. At high ionic strength 130S particles were transformed into 160S particles, and 45S into 80S particles. The polypeptide composition of each virus particle was examined. Pulse-chase experiments confirmed that 80S particles were the predominant virus particles accumulating. No precursor-product relationship could be established for the 80S particle, although 5S and 14S particles were shown to be precursors of mature virus particles.