Public awareness and concern for environmental protection and human safety has led to the search for naturally occurring compounds that are able to inhibit growth and development of weed plants. It has been reported that corn gluten meal is useful as a natural preemergence herbicide and fertilizer material for various plant production systems. The active components responsible for the herbicidal activity of corn gluten meal could potentially be used as natural herbicides;Based on the results from greenhouse and growth chamber bioassays using three species of grass plants, it was found that samples of hydrolyzed protein from corn gluten meal were more herbicidally active than the corn gluten itself and were highly water-soluble. Gluten hydrolysate prepared with bacterial proteinase had the highest inhibitory activity. This material was chosen for isolation and identification of the bioactive compounds. The response of roots was more sensitive than that of shoots to the inhibitory compounds;Five dipeptides were isolated from an aqueous solution of the corn gluten hydrolysate and their inhibitory effects on roots of test grass plants were demonstrated in petri dish bioassays. The five dipeptides were glutaminyl-glutamine, alaninyl-asparagine, alaninyl-glutamine, glycinyl-alanine, and alaninyl-alanine. These 5 dipeptides showed no synergistic effect on the root-inhibiting activity. However, the bioactivity was increased as increasing the proportion of the more active dipeptides in the mixture. In a soil bioassay, alaninyl-asparagine and glycinyl-alanine were demonstrated to inhibit rooting of creeping bentgrass;Eight other synthetically produced dipeptides were tested for their inhibitory activity using the perennial ryegrass petri dish bioassays. Of those tested, alaninyl-glycine was found to be the most inhibitory and glycinyl-glutamic acid the least. The constituent amino acids of the dipeptide were found to be more important than their sequence for bioactivity. Of the 6 amino acids, L-asparagine was the most potent root inhibitor, followed by L-alanine, L-glutamine, and glycine.