Ten avian reovirus-specific polypeptides, ranging in molecular weight from 32,000 to 145,000, were identified in chicken embryo fibroblasts infected with virulent and attenuated strains. Time-course studies on the viral polypeptides indicated that all of these proteins could be detected as early as 16 hr postinfection (PI) for both strains of avian reoviruses. With the virulent strain 1733, protein production reached a plateau 24 hr PI, whereas with an attenuated strain S1133, production plateaued at 48 hr PI. Immunoprecipitation of labeled polypeptides from S1133 and 1733 strains with homologous and heterologous sera revealed that sera against strain 1733 recognizes sigma NS protein in both strains 1733 and S1133, whereas sera against strain S1133 recognizes this protein only in 1733, suggesting a difference in the primary structure or the conformation of that particular protein in both viruses. None of the polypeptides of avian reovirus were glycosylated or phosphorylated.