An amphipathic polypeptide, Pn, with a tandemly repeated LKELPEKL sequence including a proline every eight residues, as well as a series of shorter peptides having the same sequence, P2, P3, P4, P5 and P6, were synthesized. Their conformation in aqueous solution was mainly studied by CD. At low temperature, these peptides and polypeptides are completely unordered and undergo a reversible transition leading to a partly alpha-helical structure upon heating. Such behavior has been demonstrated for a few proteins by other authors and has been called cold-denaturation. The transition temperature of the polypeptide is close to 20 degrees C. The conformational change does not depend on concentration, indicating a monomolecular process. The high-temperature structure seems to be compact as for globular proteins. A model of folded structure is proposed from experimental data and from molecular modelling studies.