The production of neutralizing monoclonal antibodies (MAbs) will permit the exact localization of neutralizing epitopes on the AIDS virus, HIV-1. We describe the properties of seven MAbs to the envelope of the LAV-1 isolate. Five MAbs recognise the central portion of gp110, amino acids 279–472, and four of these are capable of high-titre neutralization of HIV-1, by infection inhibition, syncytial inhibition and vesicular stomatitis virus (VSV) pseudotype neutralization. One of the two MAbs to gp41 inhibits syncytium formation. Neutralization, live cell immunofluorescence and immunoprecipitation of gp110 are type-specific and restricted to HIV-1 isolates closely related to LAV-1