Entry of HIV-1 into target cells requires cell surface CD4 as well as additional host cell cofactors. A cofactor required for infection with virus adapted for growth in transformed T cell lines was recently identified and named fusin. Fusin, however, does not promote entry of macrophage-tropic viruses that are believed to be the key pathogenic strains in vivo. It has now been determined that the principal cofactor for entry mediated by the envelope glycoproteins of primary macrophage-tropic strains of HIV-1 is CC-CKR5, a receptor for the .beta.-chemokines RANTES, MIP-1.alpha., and MIP-1.beta.. It has also been found that individuals who are homozygous for a mutation of the CKR-5 receptor are resistent to HIV infection; in vitro infection requires a 1000-fold higher dose of HIV than normal cells. The mutation results in complete suppression of CKR-5 expression.