We have previously described an in vitro assay for RNA-dependent RNA polymerase activity in virions of tomato spotted wilt virus (TSWV). Here we report antibody inhibition of virion-associated RNA synthesis in vitro with an L-protein-specific polyclonal antibody raised against the carboxy-terminus of the L protein. In contrast, RNA synthesis was not inhibited by a heterologous antiserum and was unaffected by antiserum raised against an internal portion of the L protein. Our results directly associate the TSWV L protein, the putative viral polymerase, with RNA synthesis functions in vitro.