Mouse L fibroblasts infected with mouse hepatitis virus, MHV3, and radiolabelled with 35S-methionine, contained, in addition to the three major structural polypeptides, p24, p56 and p180, two additional ones, p22 and p50. Of these total five polypeptides, only three, p22, p56 and p180, were labelled in infected cells during a 2 min 35S-methionine pulse and are, therefore, presumed to be immediate translation products. Pulse-chases and chymotryptic peptide mapping experiments showed apparent precursor-product relationships between p56 and p50 and between p22 and p24. Protein synthesis in infected cells was synchronized at the initiation stage by pre-exposure to hypertonic medium. Using a 0.5 min pulse-10 min chase sequence, to limit incorporation of 35S-methionine to stretches of approx. 100 amino acids adjacent to translational initiation sites, it was found that all three polypeptides, p22, and p56 and p180 contained radiolabel. It is thus apparent that translation of the three major structural proteins (or precursors) is initiated independently rather than at a single site as in the cases of other positive-strand RNA viruses such as polio or Semliki Forest virus.