This invention provides an antibody capable of specifically inhibiting the fusion of an HIV-1 envelope glycoprotein.sup.+ cell with an appropriate CD4.sup.+ cell without cross reacting with the HIV-1 envelope glycoprotein or CD4 and capable of inhibiting infection by one or more strains of HIV-1. This antibody is then used to identify a molecule which is important for HIV infection. Different uses of the antibody and the molecule are described.