Several picornaviruses appear to contain more than one polypeptide chain. When examined by polyacrylamide gel electrophoresis, poliovirus (Maizel, 1963; Maizel & Summers, 1968) and mouse encephalomyocarditis virus (Burness & Walter, 1967), each gave four bands and mouse encephalitis virus gave three bands (Rueckert & Duesberg, 1966). The most reasonable explanation for these observations is that each of the viruses contains more than one polypeptide chain; but the multiplicity of the bands could be due to chemical modification of certain amino acids in the virus protein during the experiment or to aggregation or breakdown of a single protein. Vande Woude & Bachrach (1968) considered that the multiplicity of bands they obtained from foot-and-mouse disease virus was due to aggregation of a single polypeptide. However, Wild, Burroughs & Brown (1969) used a double labelling technique and concluded that the multiple bands obtained with this virus are not due to aggregation of a single polypeptide chain.