The synthesis of virus-specific proteins in several cell types infected with Sendai virus have been analyzed by high resolution polyacrylamide-gel electrophoresis. Eight structural proteins of the virion have been identified in infected cells and the kinetics of their synthesis examined. In addition, two nonstructural proteins with molecular weights (MW) of ∼ 36,000 and ∼ 22,000 have been found. The results of pulse and pulse-chase experiments suggest that large, polyprotein precursors are not involved in Sendai virus replication, and that polypeptides are synthesized from monocistronic messenger RNA species. Throughout the replicative cycle, the virion polypeptides are synthesized in approximately the same unequal proportions at which they are present in the virus particle, except for a relatively greater amount of the P polypeptide and a smaller amount of the M polypeptide in the cells, indicating that polypeptide synthesis is strictly controlled. Results of a double-isotopic label difference analysis showed that host-cell protein synthesis declined by 8% only over 12 hr and that viral protein synthesis was superimposed upon cellular synthesis with total protein synthesis in infected cells reaching 205% of that in uninfected cells. The available evidence suggests that one of the viral polypeptides (No. 7, MW ∼ 42,000) is a cellular polypeptide and that infection selectively reduces the synthesis of this polypeptide.