A sensitive, rapid, and quantitative ELISA for p24 is described, which can be used as the read-out to test for HIV-1 neutralization in the syncytium-forming microassay and can replace the counting of syncytia under the microscope. This assay can be used reliably for divergent strains of HIV-1, including those that do not induce syncytia. The new read-out permits evaluation of neutralization at 3 days rather than 5 days. Using this assay, the neutralizing activity of several new and previously described human monoclonal antibodies against HIV-1 was characterized.