The backbone dynamics of the cytokine interleukin-8, a symmetric homodimer of overall molecular mass 16 kDa, has been investigated at pH 5.2 by means of 15N relaxation measurements using heteronuclear two-dimensional inverse detected 1H-15N spectroscopy. 15N T1, T2 and NOE data were obtained for 66 out of a total of 67 backbone amide groups. The overall correlation time is 9.10(+/- 0.05) ns at 26.6 degrees C. All residues exhibit very rapid motions on a time-scale of < or = 20 ps. These very rapid motions alone can account for the 15N relaxation behaviour of 30 residues. The 15N relaxation data for another 21 residues can only be accounted for by the inclusion of an additional internal motion on a time-scale ranging from 0.5 to 3.5 ns. These residues are clustered at the N and C termini, and in the loop regions connecting elements of secondary structures. Finally, the 15N relaxation data for another 15 residues could only be accounted for by the presence of chemical exchange on a time-scale ranging from approximately 170 ns to 2.25 ms. In addition, the inclusion of chemical exchange improved the fit to the experimental data for 10 of the 30 residues whose 15N relaxation behaviour could be accounted for by very fast motions alone. The residues exhibiting chemical exchange line broadening cluster at the interface of the long C-terminal alpha-helix and the underlying beta-sheet. It is suggested that this clustering is indicative of concerted rather than independent motions in regions of secondary structure, with motion at any one residue being propagated to neighbouring residues in van der Waals contact.