The foaming properties of rumen liquor, bovine salivary mucoprotein, cytoplasmic protein from red clover, and saponins from various pasture legumes have heen examined.Foam strengths of the saponin foams have pH optima, those from red clover and lucerne being between pH 4.5 and 5.0. The presence of calcium in the saponin is necessary for the formation of rigid foams.Foams from red-clover cytoplasmic protein show maximum strens-ths at pH values ranging from 5.4 to 6.0. The foaming properties are markedly dependent on salt concentration.Salivary mucoprotein has greatest foam strength above pH 7.5, but some samples have a secondary peak at pH 6.5.Rumen liquor from cows fed on red clover has a pH optimum for foam strength at pH 5.4 to 5.7 similar to cytoplasmic protein. The particulate matter in rumen liquor has considerable anti-foaming properties.The evidence indicates that cytoplasmic protein is of major importance as a foaming agent in causing bloat in cattle. The implications of these results are discussed with reference to bloat.