Previous studies have noted the existence of a 190,000-dalton vesicular stomatitis virus (VSV) protein called the large (L) protein. To determine whether this protein is a nonspecific aggregate, a precursor to the other VSV proteins, or a unique viral protein, its synthesis relative to the other VSV proteins was studied under conditions of inhibition of initiation of protein synthesis. Also, its tryptic peptides were compared to those of the other VSV proteins. In both cases the results were consistent with the identification of the large protein as a unique viral protein.