The foaming properties of skim milk vary with temperature of foaming in the range from 5 to 85°C, with foams of maximum stability being formed at approximately 45°C. This paper reports the significance of different milk fractions in the foam and concludes that the micellar casein fraction plays an important role in stabilization of milk foam formed at higher temperatures. This finding was supported by the fact that added calcium chloride increased and calcium-chelating agents decreased foam stability. These effects were attributed to the increase and decrease, respectively, in the amount of micellar casein in the milk. Furthermore, bubble ghost material sedimented by low-speed centrifugation of foam was found to contain predominantly caseins, and electron micrographs of foams formed at 45°C clearly showed casein micelles spread over the interface. However, other structures observed in the electron micrographs suggest that soluble milk proteins and possibly polar lipids are also present in the foams and play a role in formation of milk foams.