Annexin II (p36) interacts with its ligand p11 via the short stretch of 12 amino acids (Ac-S-T-V-H-E-I-L-C-K-L-S-L) situated at the N-terminus. We have now synthesized some 37 tetradecapeptides, which differ from the original p11 binding sequence (Ac1-14) by single amino acid substitutions. The relative affinity of each peptide for p11 was determined by fluorescence spectroscopy using a competitive binding assay. The binding behaviour of the different peptides confirms the model of an amphiphilic alpha-helix induced upon binding to p11. The apparent affinities delta delta Gbind of the mutant peptides revealed that the N-acetyl group of serine 1 and the hydrophobic side chains at positions 3, 6, 7 and 10 contribute most to the binding. The observed destabilization of the complex upon removal of signal methyl groups from the hydrophobic side of the helix is comparable with the destabilization of proteins in which methyl groups have been removed from the inner core. We conclude that upon binding to p11 the hydrophobic side of the amphiphatic alpha-helix becomes fully buried.