An antiserum raised against TYMV-RNA encoded protein P115 partially inhibits TYMV RNA replicase activity, demonstrating that this protein is involved in TYMV RNA synthesis. The detection of protein P115 by an antibody linked polymerase assay demonstrates that protein P115 is indeed a subunit of the TYMV RNA replicase, the enzyme known to synthesize viral RNA in infected Chinese cabbage. The use of translation products of other tymoviruses indicates that the serological relationship between the virus-encoded replicase subunits of these viruses and protein P115 is very weak at the best.