Stabilization of triglyceride emulsions by proteins was studied in vitro using either chemical or optical methods to measure to amount of fat emulsified under standard conditions of emulsification. Concentrations, pH, and degree of digestion of protein emulsifiers were varied to determine how such manipulations affected emulsion stabilization. Proteins from endogenous as well as exogenous sources were capable of stabilizing triglyceride emulsions, and this effect varied with protein concentration. Peptic digests of a prototype protein retained the efficacy of the undigested protein, while further digestion of the protein in pancreatic proteases decreased, but did not abolish, the emulsifying power of the protein. Emulsion stabilization by peptic digests of protein still persisted despite alterations in pH from 1 to 7. Such emulsions were capable of being hydrolyzed by pancreatic lipase and could be absorbed from the gut of bile-deficient rats. These emulsions also speed the initial rates of fat absorption from animals with an intact bile supply. We conclude that stabilization of luminal emulsions by dietary or endogenous protein may facilitate fat absorption similar to the way that detergent-stabilized emulsions have been previously shown to have these effects.