The chemokine receptor CXCR4 was solubilized from the human T-cell line CEM by using the detergent n-dodecyl-beta-maltoside (DDM) and cholesteryl hemisuccinate ester (CHS). Binding studies with I-125-SDF-1 alpha revealed a dissociation constant of 5.33 nM and a receptor density (B-max) of 2.68 pmol/mg in CEM membranes at 4 degrees C. The affinity of solubilized CXCR4 for SDF-1 alpha was identical to membrane-bound CXCR4. Binding of gp120 to solubilized CXCR4 was demonstrated by coprecipitation of gp120 with anti-CXCR4 antibodies.