Extensive studies of the proteins of particles of enteroviruses, cardioviruses and foot and mouth disease (FMD) virus (Holland & Kiehn, 1968; Maizel & Summers, 1968; Rueckert, Dunker & Stoltzfus, 1969; Burroughs et al. 1971; Johnston & Martin, 1971; Talbot & Brown, 1972) indicate that these groups of picornaviruses are similar in having three major and one minor polypeptide. Published results on rhinoviruses have been conflicting. Medappa, McLean & Rueckert (1971) found that rhinovirus (RV) 1A contained polypeptides similar in size and number to those of ME virus. In contrast, Korant, Lonberg-Holm & Halperen (1970) originally reported that human rhinovirus types 2 and 14 contained only three major polypeptides and thus differed from enteroviruses. However, re-examination of these rhinovirus by Korant et al. (1972) has shown that they do contain the small polypeptide.