The effect on .beta.-lactoglobulin foam-ability and foam stability of the poly(ethylene oxide)-poly(propylene oxide) block copolymers F127 (PEO99-PPO65-PEO99), mol. wt. 12500 g/mol, and P85 (PEO26-PPO39-PEO26), mol. wt. 4600 g/mol, has been investigated at const. protein concn., 10 .mu.M (0.2 mg/L), and varying block copolymer concns., ranging from 0.02 to 1600 .mu.M. Foam was generated by means of air sparging and the foam vol. and liq. vol. of the foam were measured for one hour. It was found that foam stabilized by F127 or P85 in the concn. range 20-1600 .mu.M contained a larger liq. vol. initially than pure ss-lactoglobulin foam. Furthermore, ss-lactoglobulin foam-ability was only marginally affected by the presence of F127, while it was reduced in an interval of low P85 concns. The protein foam stability was retained in the presence of the larger polymer F127, whereas P85 largely reduced the stability, indicating that the size of the polymeric surfactant is important. The results are discussed in relation to surface rheol. properties and forces acting across foam films. Steric repulsion generated between the surfaces of foam films is suggested to be the main stabilizing factor in dry foam contg. F127. The instability of the mixed ss-lactoglobulin/P85 system is suggested to be caused by two effects. First, there are incompatible stabilization mechanisms of block copolymer and protein, as supported by previous surface rheol. data. Second, there is a reduced importance of long-range steric repulsion when P85 is added, compared to the case where F127 and ss-lactoglobulin are mixed.