The compressive strength and persistence of protein foams produced from extracts of legume leaves were measured. The presence of foliar proanthocyanidin (PA, condensed tannin) in species such as Onobrychis and Lotus correlated with foams of negligible compressive strength and persistence and also with a reputation for bloat safety in the field. PA purified from forage legume leaves significantly decreased the compressive strength of protein foams produced from purified red clover leaf protein. The decrease in the compressive strength was related to the concentration of added PA in a dose-dependent manner. A ratio of PA to protein of 0 . 1 reduced the compressive strength by approximately half relative to the strength of the PA-free control. Purified PAS with different chemical structures had similar effects on the compressive strength of protein foams. The significance of these observations to pasture bloat and forage legume improvement is discussed.