A program of investigations is outlined and the results of the first three years' work reported. Eight wheat varieties, differing widely in resistance to stem rust, showed no corresponding differences in the physico-chemical properties of their expressed tissue-fluids. The infection of susceptible varieties was in some cases reduced by administering extracts of resistant varieties in petri-dish cultures or by direct injection into inoculated leaves. The injection of the juice of infected leaves into healthy leaves failed to demonstrate the presence of any toxin excreted by the fungus. The injection of salicylic acid, catechol or vanillin in suitable concentrations frequently caused a reduction in infection. These phenolic compounds in very low concentrations stimulated the growth ofHelminthosporium sativum, but at higher concentrations inhibited it. The same phenols inhibited the germination of rust spores. On filtered wheat-juice rust spores also failed to germinate, though on unfiltered juice they germinated normally.