Viral messenger RNA was isolated from BHK cells infected with a temperature-sensitive mutant of Sindbis virus and was further purified using an oligo(dT) column. Addition of this mRNA cell-free extracts from rabbit reticulocytes led to formation of discrete authentic viral capsid protein when the reaction was performed at 29 C. However, this same protein-synthesizing system failed to make discrete viral capsid when incubated with the viral RNA at 39 C. Instead, larger-molecular-weight polypeptides that contained the viral capsid peptide sequences were produced. The inability to make a separate viral capside protein in vitro at elevated temperatures by the mRNA from this mutant exactly mimics the phenotype of this ts mutant in viral-infected cells. Three mechanisms are discussed that might account for a temperature-sensitive release of capsid. One of these is based on a model in which there are multiple sites for initiation of translation of polypeptides on a polycistronic viral mRNA.