This protocol describes a simple assay that can be used to study the nature of exposure of antigenic epitopes and antigenic relatedness of different intact, native HIV-1 strains. The assay is based on the principle that mAbs coated on microtiter wells bind to epitopes on the surface of intact, native virions. The bound virion is then lysed to release p24, which is then quantitated (pg/ml) to give a measure of the amount of virion bound to the mAb. High p24 levels released after lysis correlate with high level capture of virions by mAbs, and as such, reflect good exposure of the epitope on the virion. Likewise, binding patterns of a specific mAb with different virus strains reveal information on their antigenic relatedness. In establishing this assay, the nature of exposure of antigenic epitopes and the antigenic relatedness of six intact, native HIV-1 virions of clades A, B, C, D, F and G were examined using anti-HIV-1 mAbs directed at epitopes in the V2, V3, CD4bd and C5 of gp120, and in clusters I and II of the gp41 region. Analysis of the binding data shows that mAbs directed at epitopes in the V3, C5 and gp41 Cluster I region bound best to the viruses examined, suggesting that these are the regions most exposed and conserved on intact, native HIV-1 virions of different clades. Epitopes in the V2 and CD4bd of gp120, and in gp41 cluster II, are not exposed on intact, native virions.