The binding affinity for a 12-bp dsDNA of Antennapedia helix 3 analogues, major groove binders, has been measured by displacement of prebound ethidium bromide, a fluorescent displacement assay proposed for minor groove binders by Boger et al. (J. Am. Chem. Soc., 2000, 122, 6382–6394). Relative binding affinities determined by this method were compared to those obtained by gel mobility shift and footprinting assays for the 12-bp dsDNA and a 178-bp DNA fragment. The present work demonstrates that the fluorescence displacement assay is suitable for rapid screening of major groove binders, even though about 60 to 70% of the prebound ethidium bromide is displaced by these peptides. Total (100%) displacement of ethidium bromide was serendipitously achieved by addition in the peptide sequence, at the N-terminus, of a S-3-nitro-2-pyridinesulfenyl-N-acetyl-cysteine residue. S-3-nitro-2-pyridinesulfenylcysteine was shown to (i) bind to dsDNA with a micromolar affinity and (ii) direct within DNA grooves a peptide with no affinity for dsDNA.