The protein composition of several members of the animal picornavirus group is remarkably similar. Thus, the enteroviruses polio type 1, Echo 12 and bovine entero VG-5-27 (Maizel & Summers, 1968; Korant, Lonberg-Holm & Halperen, 1970; Johnston & Martin, 1971), the cardioviruses Maus-Elberfeld and encephalomyocarditis (Rueckert, Dunker & Stoltzfus, 1969; A. T. H. Burness, personal communication), the rhinoviruses 1A and HGP (Medappa, McLean & Rueckert, 1971; E. J. Stott & R. Killington, personal communication) and several of the seven immunological types of foot-and-mouth disease virus (Burroughs et al. 1971; P. Talbot, unpublished observations) contain three polypeptides with molecular weights ranging from 37 to 24 × 103 (VP1, VP2, VP3) and a smaller polypeptide with a molecular weight in the range of 7 to 13.5 × 103 (VP4). In several instances the molar ratio of the four polypeptides has been determined and, with the exception of rhinovirus 1A (Medappa et al. 1971), the three larger polypeptides are present in equimolar amounts and the smaller polypeptide is present in a half molar amount relative to the others.