The effects of protein addition (lysozyme and bovine serum albumin) on surface tension reduction and cloud point of a surfactant (Pentaethyleneglycol Mono n-Dodecyl Ether (C12E5)) were studied. The minimum surface tension was in all cases 27 mN/m but was reached at a higher surfactant concentration for the BSA/ C12E5solution than for the pure surfactant or the lysozyme/surfactant mixture, indicating that C12E5binds to BSA but not to lysozyme. It was estimated that BSA bound four to five molecules of C12E5. Only BSA seemed to have any large effect on the cloud point of the surfactant and above a certain critical protein to surfactant ratio the clouding is totally suppressed. The results implies that the presence of protein influences the clouding only if surfactant binds to the protein.