Maturation of virus-specific hemagglutinin and neuraminidase polypeptides was studied in chick embryo fibroblasts infected with influenza A virus, the WSN strain. Hemagglutinin maturation is accompanied by a decrease in the electrophoretic mobility of the protein. The processes occurring in maturation are localized on the part of the molecule corresponding to the major hemagglutinin subunit. The previously reported virus-induced proteins III and IV have been identified as immature precursor and mature polypeptide of virus neuraminidase, respectively. This is concluded on the basis of the membrane localization of both proteins, the presence of sugars in at least one of them, the antigenic specificity of the proteins, their behavior in electrophoresis under nonreducing conditions, as well as the results of "pulse'chase" label experiments. The nature of the modifications accompanying viral neuraminidase maturation remains obscure; some possibilities are considered.