Red clover has high protein levels at harvest that are maintained during ensiling. Decreased proteolytic activity in red clover is due to polyphenol oxidase (PPO) activity and key o-diphenol substrates. This project was undertaken to determine if PPO activity and appropriate o-diphenols are present in a range of grasses and their potential role in proteolytic inhibition. Ten cool season grasses were established in the greenhouse and field plots in 2003-04. Leaf blades were harvested from field and greenhouse plots, frozen in liquid nitrogen, and stored at –80 °C until processed for PPO activity, o-diphenols, browning, and proteolytic inhibition. The amount of PPO activity measured in each grass varied depending upon the species and upon the specific type of o-diphenol used as the primary PPO substrate. Orchardgrass, meadow fescue, ryegrass, and smooth bromegrass exhibited the highest PPO activities (110, 35, 26, 8 x10^-4mmoles/mg/min respectively) with caffeic acid as substrate. Chlorogenic acid and/or caffeic acid were the preferred substrates, although there were differences among the most active grasses as to which was the best utilized. Characterization for common o-diphenols caffeic acid and chlorogenic acid revealed these were present in high concentrations in grasses with low PPO activity and were typically absent from those that had high PPO activity. All grasses with high PPO activity underwent rapid browning (an assay typically used to indicate the presence of PPO activity) possibly indicating other types of o-diphenols present in these plants. Generally, the addition of caffeic acid to isolated grass extracts resulted in proteolytic inhibition in grasses with measurable PPO activity. Such results suggest that several important grass species contain PPO activity, but may lack the appropriate o-diphenol substrates to effectively inhibit proteolysis.