The strain-specific serological activity of 2 strains of tobacco mosaic virus (TMV) and 2 strains of clover yellow mosaic virus (CYMV) was confirmed by complement fixation tests. Eleven of 12 strains of potato virus X (PVX) proved to be serologically indistinguishable. One of the mottle strains was shown to differ from the other mottle and ringspot strains in tests with cross-absorbed antisera.