The avian viral agent S1133 has previously been classified serologically as a member of the avian reovirus group. This viral agent grows in chicken embryo fibroblast cells, bands at a density of 1.37 g/ml in CsCl equilibrium density gradients, has a particle diameter of 75 nm, and has a morphology similar to that of human reovirus type 3. Its nucleic acid is comprised of double-stranded RNA and adenosine-rich oligonucleotides. The dsRNA is distributed among 10 segments with molecular weights of 2.7 x 10(6), 2.6 x 10(6), 1.7 x 10(6), 1.5 x 10(6), 1.3 x 10(6), 1.2 x 10(6), 0.80 x 10(6), 0.74 x 10(6), and 0.68 x 10(6) for the largest (L1) to the smallest (S4) segment, respectively, as determined by polyacrylamide gel electrophoresis. These 10 segments migrate differently on polyacrylamide gels compared to those of human reovirus type 3. The capsid proteins of avian reovirus consist of eight species of polypeptides as determined by polyacrylamide gel electrophoresis. These are lambda1, lambda2, lambda3, mu1, mu2, sigma1, sigma2, and sigma3 with molecular weights of 140, 125, 115, 85, 72, 40, 36, and 32 x 10(3), respectively. Only polypeptide sigma2, which resides in the inner capsid or core, comigrated with the sigma2 polypeptide of type 3 reovirus. Antiserum against type 3 reovirus did not neutralize avian reovirus. Avian reovirus core particles were found to possess a transcriptase and a methylase activity.