Cytoplasmic extracts of rhinovirus 1A-infected HeLa cells, pulsed 15 min with [3H]leucine, contained a 13S subunit which was rich in the capsid precursor, peptide 92. After a 30-min chase, most of the capsid-related protein sedimented in a 14S peak that contained equimolar amounts of the capsid peptides epsilon, alpha, and gamma, and some residual chain 92. The 14S subunit could be dissociated at pH 4.8 into 6S subunits containing only epsilon, alpha, and gamma chains in equal proportions, indicating that the 14S subunit is an oligomer of (epsilon gamma alpha) protomers. These subunits resemble subunits previously identified in the assembly of enteroviruses. These observations support the idea that rhinovirus assembly is basically similar to that of enteroviruses. Comparative studies on the peptide stoichiometry of the virion and the capsid precursor subunits indicate that rhinovirus 1A can contain as many as 11 immature protomers per virion.