A sequence variant of human MIP-1α, in which Asp26 has been replaced by Ala, has been chemically modified by the addition of 13C-labeled methyl groups at each of the lysine residues and the N-terminus. The sites of methylation have been verified by a combination of MALDI-TOF mass spectrometric experiments and tryptic digestion followed by N-terminal mapping. The effect of the modification on the structure and activity of the protein have been determined by analytical ultra-centrifugation, 13C NMR spectroscopy and receptor binding studies. The results of these experiments suggest that huMIP-1α D26A (BB10010), when present as a dimer, adopts a globular structure, like MCP-3, rather than the elongated or cylindrical structure determined for dimers of huMIP-1β and RANTES.