The virus-specific proteins in three types of cultured cells infected with the WSN strain of influenza virus have been analyzed. The seven structural proteins of the WSN virion were identified in infected cells, and in addition a nonstructural protein, designated NNP, was found in large amount in the nucleus of the infected cell. This nonstructural protein has an estimated molecular weight of ∼25,000 and is slightly smaller than protein 7, the smallest viral structural protein. Proteins NNP and 7 were isolated on an agarose column and shown to be distinct by peptide mapping. The seven viral structural proteins have been found in association with the plasma membrane of infected cells. Glycoproteins 5 and 6 are the last proteins to appear at the plasma membrane. The available evidence suggests that protein 2, which is found in much larger amount in infected cells than in the mature virion, is cleaved at the plasma membrane to yield proteins 5 and 6.