The rate of formation and properties of a viral-specific cytoplasmic fraction which sediments at 140 S and is produced in Sindbis-infected BHK cells have been studied. The fraction contains 42 S infectious viral ribonucleic acid (RNA) and it accumulates in infected cells. The rate of formation of 140 S material is similar to the rate of release of Sindbis virions from cells. The 140 S particle is not infectious under conditions where infectivity of virions or infectious RNA may be demonstrated. These properties agree with the concept that the 140 S particle is a nucleocapsid precursor of the virion. A second cytoplasmic fraction which sediments at 65 S contains viral-specific but non-infectious RNA.