Denaturation of bovine beta-lactoglobulin (beta-L) in pH 2.9, 0.2 M glycine buffer was investigated by DSC in the presence of three nonionic surfactants, n-octyldimethylphosphine oxide (APO8), n-decyldimethylphosphine oxide (APO10), and n-dodecyldimethylphosphine oxide (APO12), and by ITC at temperatures from 25 to 61 degrees C. The thermal transition was eliminated when the molar ratio of surfactant to beta-L was between 88 and 133, 21-25, and 11-22 depending upon the protein concentration for APO8, APO10, and APO12, respectively. A protocol was developed that may be used for future studies that involve ligands with large temperature-dependent heats of dilution. Approximately 30 mol of APO10 and APO12 per mole of beta-L were bound at 45 degrees C and 37 degrees C, respectively, with an average affinity of (2.5 +/- 0.7) x 10(3) M-1. This amount of surfactant would cover about 50% of the protein surface and may correspond to a new class of nonspecific neutral ligand binding sites that facilitate the digestion of lipids by neonatal calves. Titration of beta-L into 2% solutions of APO8, APO10, and APO12 at various temperatures between 25 and 61 degrees C yielded enthalpy changes with the same temperature dependence as for the thermal denaturation of beta-L without surfactant at much higher temperatures.