OBJECTIVE: To evaluate human monoclonal antibodies (MAb) for neutralizing activity against primary HIV-1 isolates in peripheral blood mononuclear cells. DESIGN: Neutralization activity data were obtained from 11 laboratories on a coded panel consisting of six human MAb to HIV envelope V3, CD4-binding region or gp41. Hyperimmune globulin against HIV-1 and normal human immunoglobulin G were supplied as controls. Each laboratory received pre-titered virus for use in the studies. METHODS: Each laboratory measured neutralization of the MAb against laboratory strain HIVMN, genomic clone HIVJR-CSF, two subtype B and one subtype D primary isolates. RESULTS: The titers of the centrally supplied virus stocks as determined by re-titration or back-titration varied among laboratories and were generally 10-100-fold less than provided. The neutralizing activity of each MAb varied by as much as a 1000-fold among laboratories. These differences may result from varying sensitivity in neutralization assay protocols and the differing susceptibility of primary cells to infection with HIV-1. CONCLUSIONS: To consolidate the data from multiple laboratories, the neutralization titers were compared by classifying antibodies as neutralizing if the antibody concentration for 50% virus inhibition was < or = 10 micrograms/ml. By this criterion, the CD4-binding region and gp41 MAb neutralized all four subtype B viruses and the subtype D isolate in a few of the laboratories. The V3 MAb neutralized only HIVMN and the closely related HIVJR-CSF viruses.