SUMMARY A bovine enterovirus (serotype VG-5-27 ) was grown in BHK 21 cells and purified using gel filtration and sedimentation. A minor component which sedimented at i45 S, relative to 165 S virus particles, was purified by repeated sedimentation on sucrose density gradients. The I45S particles contained RNA of the same size as virus RNA. Five types of polypeptides were present corresponding to 3o VPo, 60 VPI, 3 ° VP2, 60 VP3 and 30 VP4 chains, in contrast to I65S virus particles which contain 6o VPI, 60 VP2, 60 VP 3 and 30 VP4 chains. The particles appeared to be non-infective but could be isolated from early passages of plaque purified clones of the virus. Iodination of the polypeptides of the 145 S component differed considerably from that for mature virus particles. We suggest that the particle may be an intermediate in the maturation of the virus particle.