AbstractRaspberry leaves contain a substance which prevents the infection of plants by viruses when it is mixed with the inoculum. It is lost on dialysis, precipitated by nicotine or added protein, unaffected by boiling or freezing, and is probably a phenolic tanning agent. Extracts from peach or apple leaves contain less of such substances than do those from leaves of plum, cherry, raspberry or strawberry. The degree to which virus infection is inhibited by tanning substances from raspberry or other sources depends on the virus and not on the species of test plant. Raspberry tannin and tannic acid combine irreversibly with some viruses but with others the combination is readily reversed by dilution or increase in pH value. With potato ring necrosis, tobacco mosaic and tobacco necrosis viruses, there seems to be a fixed ratio between the amount of virus and the amount of tannin needed to decrease infectivity by a fixed proportion. The infectivity of extracts made from raspberry leaves containing raspberry ring spot or beet ring spot viruses varied with the concentration of tannin present and the pH value. The amount of virus sedimented by centrifuging such extracts at low speed decreased with increase of pH value. Extracts made with 2 % nicotine in water were usually more infective than those made with alumina or phosphate buffer (pH 8), and much more infective than those made with water. Both viruses are precipitated by acetone or ammonium sulphate from extracts of infected raspberry leaves made with nicotine or alumina; beet ring spot virus was detected serologically in such preparations from infected raspberry leaves.