Apomyoglobin was chosen as a model to study the emulsifying properties of proteins. It was cleaved into three peptides using cyanogen bromide and these peptides were purified and tested for emulsifying and interfacial properties. Two of the peptides had improved emulsifying activity compared to the whole protein. The peptide (residues 1-55) with the highest emulsifying activity and whole apomyoglobin were studied further. The amount of protein or peptide adsorbed at the oil-water interface of an emulsion was measured and the surface area occupied per molecule was calculated. For apomyoglobin, at maximal surface coverage each molecule occupied a surface area of ϳ8 nm 2 . This is consistent with a packed monolayer, based on the approximate dimensions of apomyoglobin. For peptide (1-55), at maximal surface coverage each molecule occupied a surface area of ϳ3 nm 2 . This is consistent with the area that the two amphipathic ␣-helices which are potentially present in this molecule would cover, if they were aligned along the oil-water interface. The different structural characteristics of these molecules responsible for their emulsifying properties are discussed.