The invention relates to SAEP II peptide dimers that mimic polymyxin B i.a. in its ability to bind non-covalently the lipopolysaccharide (LPS) of Gram-negative bacteria with high affinity, and therefore to detoxify LPS. The dimeric structure is maintained by a pair of disulphide bonds between two cystein residues present in the peptide sequence, which does not exceed 17 amino acids and essentially comprises cationic and hydrophobic amino acid residues. The peptides in the dimers may have a parallel or anti-parallel orientation. SAEP II dimers are useful for treating or preventing septic shock and related disorders generated by Gram-negative bacteria infection. The invention also relates to LPS-peptide complexes in which LPS and SAEP II diners are non-covalently bound together. These complexes are useful as vaccinal agents against Gram-negative bacteria infection.