Stromal-cell-derived factor-1 (SDF-1 alpha) is an 8-kDa chemokine that is constitutively expressed in bone-marrow-derived stromal cells and has been identified as a ligand for the CXCR4 receptor. We produced the chemokine recombinantly as methionine-SDF-1 alpha in Escherichia coli without the leader peptide sequence. The protein was denatured, refolded, and further purified by reversed-phase HPLC. SDF-1 alpha was shown to be >95% pure as judged by SDS-PAGE, The final yield of purified and refolded SDF-1 alpha was 1-2 mg per gram of wet cell paste. The refolded protein is a ligand for the CXCR4 receptor and has been used to block HN-mediated cell fusion and downmodulates the CXCR4 receptor. Our ability to purify hundreds of milligrams of refolded protein allowed us to conduct detailed studies of the biophysical properties of the protein. We have used a combination of biophysical techniques to study the solution properties of SDF-1 alpha, The average mass of SDF-1 alpha, as determined by static light scattering, gave us the first indications that the chemokine may self-associate. Further investigation with sedimentation velocity ultracentrifugation confirmed the existence of two species, The measured s(20,W) values defined two masses corresponding to monomer and dimer. Finally, sedimentation equilibrium ultracentrifugation and dynamic light scattering yielded a composite value of 150 +/- 30 muM for the dimerization constant. We conclude that SDF-1 alpha exists in a monomer-dimer equilibrium.