Colicin E9 is a 61 kDa antibacterial protein secreted by E. coli. In order for it to enter the cytoplasm of susceptible bacteria and kill them by hydrolysing their DNA, the colicin must first interact with an outer membrane receptor on the target cell, BtuB, and a translocation pathway involving Tol proteins. The receptor binding, translocation and DNase functions of colicin E9 are housed in discrete structural domains, which have been independently expressed and characterized. The minimal receptor-binding domain is a 76 amino acid protein (min-R). X-ray structure determination of a related colicin shows its receptor-binding domain to have a helical hairpin structure (S. Soelaiman, K. Jakes, N. Wu, C. Li and M. Shoham, Molecular Cell, 2001, 8, 1053). Our solution NMR studies of min-R have confirmed it has a helical hairpin structure, and shown it has multiple slowly interchanging conformers and a flexible inter-helix loop. A plausible interpretation of these data is that in solution the helical hairpin can adopt a variety of structures differing in the spatial relationship of the two helices. A possible biological role for this involves the hairpin opening during translocation into bacteria.