PRIOR infection of a plant with a virus may prevent or interfere with the production of local lesions or systemic infection of a strain of the same virus1. This interference or cross-protection is used as a criterion for relating plant viruses. The experiments briefly reported here are part of a study on interference between serologically related viruses although sero-logically unrelated viruses were included to observe the specificity of the reaction. Interference occurred not only between the related but also between the apparently unrelated viruses.