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SUMMARY: Evidence is given that potato paracrinkle virus, potato virus S and carnation latent virus are serologically related and should be considered as related virus strains, although they differ considerably in their host range and pathogenicity, and only carnation latent virus is transmissible by aphids. It is suggested that the three have evolved from a common aphid-transmitted ancestor. In addition to the antigens which the three have in common, each has many specific ones, and the two strains from potato are more closely related to one another than to the carnation virus. No plant of the potato variety King Edward was found free from paracrinkle virus, and no Arran Victory plant free from virus S. Minor variants of both paracrinkle virus and of virus S were detected; it is suggested that the variations in severity of symptoms developed when Arran Victory plants are grafted with King Edward scions reflect the various degrees to which different isolates interfere with each other’s multiplication. Most isolates of virus S interfere only slightly with the multiplication of paracrinkle virus.