US Patent No. 5,030,268 discloses that corn gluten meal, the protein fraction of corn (Zea mays L.) grain, can be used as a natural preemergence herbicide. However, corn gluten meal is insoluble in water, and this characteristic renders it difficult to apply as a herbicide. To seek a watersoluble material with more potent herbicidal activity, the phytotoxicity of various samples derived from corn gluten meal and other related crop materials were evaluated by using three different grass species under controlled environments. Greenhouse and growth chamber bioassays showed that the sample of enzymatically hydrolyzed corn gluten meal was more herbicidally active than the corn gluten itself and was highly water soluble. Gluten hydrolysate prepared with bacterial source proteinase had the greatest inhibitory activity to the root growth of germinating seeds. This water-soluble material derived from corn gluten meal had a growth-regulating effect on the root system and can be used as a natural herbicide.