Chemokine receptor CXCR4 (also known as LESTR and fusin) has been shown to function as a coreceptor for T-cell-tropic strains of human immunodeficiency virus type 1 (HIV-1). We have developed a binding assay to show that HIV envelope (Env) can interact with CXCR4 independently of CD4 but that this binding is markedly enhanced by the previous interaction of Env with soluble CD4. We also show that nonglycosylated HIV-1SF-2 gp120 or sodium metaperiodate-treated oligomeric gp160 from HIV-1451 bound much more readily to CXCR4 than their counterparts with intact carbohydrate residues did.