Summary The polypeptides of cells which had been infected with adenovirus and pulse labelled with high specific activity [35S]-methionine have been examined by polyacrylamide gel electrophoresis followed by autoradiography. Five virus-particle polypeptides and at least five other polypeptides which appeared to be specific for the infected cell could be discerned. One of the latter polypeptides could be detected very early in infection and is shown to be one of the major components of the previously described P antigen. The experiments also show that in the absence of arginine in the tissue culture medium, the infected cells fail to synthesize the arginine-rich core polypeptide.