An antiviral principle (AVP) has been isolated from tomato plants systemically infected with tobacco mosaic virus (TMV). This principle when mixed with TMV inoculum, or applied separately to the host before inoculation, markedly reduces the infectivity of the virus. Induction of the principle in tomato begins during the very early stages of virus–host interaction; later, with the development of a high level of AVP, there is a concomitant decline both in the amount of virus in the diseased plant and in the infectivity of the virus which is present. The production of AVP is not accompanied by overt signs of hypersensitivity or necrosis in the susceptible tomato host.