In reovirus-infected cells, virus-specific particles of different sizes accumulate that have associated with them a polymerase that copies polyribocytidylate (poly C) to yield the double-stranded poly C-polyriboguanylate in vitro. Two kinds of the different particles have the size and configuration of empty reovirus particles. Their protein composition however differs from that known for top component and complete reovirus as two proteins, mu 1 and sigma 2 are present in excess amounts. A third enzymatically active component is much smaller and has a structure which is rosette in configuration comprised of six identificable units arranged as the petals of a flower around a central hole. Each of the petals contains three distinct subunits. The major proteins present in purified preparations of these flower-like components are lambda 1 and/or 2, mu 1, mu 2, mu O, and sigma 2 and/or 2a. Some sigma 3 is present in all fractions. From the fact that no mu O or sigma 2a are present in the large empty particles which are also enzymatically active, it is likely that the poly C polymerase present in the flower-like structures consists of one or more of the following proteins: lambda 1 and/or 2, mu 1, mu 2, and sigma 2. Of these proteins, all except mu 2 are core prteins, and even mu 2 is derived by cleavage from the core protein mu 1.