The peptide Pro130–Thr–Ser–Thr–Pro–Thr–Ile–Glu–Ala–Val–Glu140–Ser–Thr–Val–Ala–Thr–Leu–Glu–Ala–Ser–Pro150–Glu–Val–Ile, which corresponds to residues 130–153 of κ-casein B, was synthesized and the conformation of the peptide in solution investigated by circular dichroism (CD) spectroscopy, structure prediction algorithms and 1H-nuclear magnetic resonance spectroscopy. In a solution containing the structure-enhancing solvent trifluoroethanol the CD spectrum was typical of a peptide in the α-helical conformation and nuclear magnetic resonance showed that the amino acids between Ile136 and Ser149 (κ-casein numbering) were predominantly in the α-helical conformation, but that Pro130 to Thr135 and Pro150 to Ile153 were not. In addition, Thr133–Pro134 and Ser149–Pro150 were primarily in the trans conformation, the residues from Thr131 to Thr135 were in unordered structures and the residues from Glu151 to Ile153 were in an extended conformation. Residues Glu137 to Glu140 and Thr145 to Ala148 also displayed some 310-helix character. When the peptide was dissolved in 10 mM-cetyltrimethylammonium chloride solution at pH 6, the CD spectra indicated that the proportion of helical structure was comparable to that of the peptide in trifluoroethanol solution (400 ml/l), whereas when the peptide was dissolved in buffer alone or in 10 mM-SDS solution, the CD spectra were consistent with a low helical content. Acidification of these solutions to pH 2·85 resulted in a slight increase in the helical content of the peptide in buffer and more markedly in buffer containing SDS. When the peptide was in 5 mM-CaCl2 solution at neutral pH, the CD spectrum indicated that some ordered structure was present. Taken together these results indicate that the ionizable residues Glu137, Glu140, Glu147 and Glu151 could be important in determining the stability of the putative helix. The structure predictions found that the sequence from Glu137 to Pro150 would be more likely to be in a helical than any other conformation in the intact bovine protein, but that pig, sheep and goat κ-caseins did not give a prediction of a strongly helical region in this part of the molecule.