This invention provides: agents determined to be capable of specifically inhibiting the fusion of a macrophage-tropic primary isolate of HIV-1 to a CD4+ cell, but not a T cell-tropic isolate of HIV-1 to a CD4+ cell; and agents determined to be capable of specifically inhibiting the fusion of a T cell-tropic isolate of HIV-1 to a CD4+ cell, but not a macrophage-tropic primary isolate of HIV-1 to a CD4+ cell. This invention also provides: agents capable of specifically inhibiting the fusion of a macrophage tropic primary isolate of HIV-1 with a CD+ cell susceptible to infection by a macrophage-tropic primary isolate of HIV-1; and agents capable of specifically inhibiting the fusion of a T cell-tropic isolate of HIV-1 with a CD4+ cell susceptible to infection by a T cell-tropic isolate of HIV-1. The agents include but are not limited to antibodies. This invention further provides: methods of inhibiting fusion of a macrophage-tropic primary isolate of HIV-1 with a CD+ cell susceptible to infection by a macrophage-tropic primary isolate of HIV-1 which comprises contacting the CD4+ cell with an amount of an agent capable of specifically inhibiting such fusion so as to thereby inhibit such fusion; and methods of inhibiting fusion of a T cell-tropic isolate of HIV-1 with a CD4+ cell susceptible to infection by a T cell-tropic isolate of HIV-1 which comprises contacting the CD4+ cell with an amount of an agent capable of specifically inhibiting such fusion so as to thereby inhibit such fusion.