A water-soluble substance isolated from the leaves of seven plant species is shown to inhibit the fungal hydrolytic enzymes, pectinase and cellulase. Three plants-sericea, muscadine grape, and persimmon-were found to be good sources of the inhibitor. Preparations of all three, isolated by the caffeine-complex method, were rated about equal in enzyme inhibition, and their chemical and physical characteristics were found to be essentially the same. When sericea was harvested in the field with a silage cutter, inhibitor activity was rapidly lost as compared to harvesting by cutting the whole stalk. Total tannin content did not appreciably change under any of the conditions of harvesting or dehydration. Grape-leaf and sericea-enzyme inhibitor are both of high molecular weight (14,000-20,000) and have certain chemical reactions that place them as condensed polymers of catechins or leucoanthocyanidins.