The binding of mAbs to intact virions was determined with a capture assay as previously described [71] . Briefly, a 96-well plate was coated overnight at 4°C with goat anti-human immunoglobulin G (IgG) Fc Abs (Sigma) at 4.0 μg/ml, and then anti-V3 human mAbs at a saturating level of 10 μg/ml were added for 1.5 h incubation at 37°C. The plate was blocked with 0.5% BSA in phosphatebuffered saline (PBS) containing 10% goat serum and 10 μg of human IgG/ml. The culture supernatant, containing pseudotyped viruses (Du156.12 and JRFL) at a p24 concentration of 25 ng/ml, and the two fold diluted primary isolate (SF162) with a starting p24/ml concentration of 50 ng were added. The plate was incubated overnight at room temperature. Viruses captured by immobilized mAbs were lysed with 1% Triton-X in PBS. Between each step of the assay, the plate was washed with PBS containing 0.05% Tween 20, pH 7.4. The p24 in the virus lysate was quantified by using a commercial ELISA. The anti-V3 Ab 447-52D and SF162, a subtype-B virus known to be sensitive to most of anti-V3 Abs were used as assay control while the human mAb, 1418, was used as a negative control.