Treatment of HIV-1 with CAP enhances the expression of binding sites for mAb NC-1. Purified HIV-1 IIIB (6.8 × 109 virus particles) and HIV-1 BaL (2.5 × 109 virus particles), respectively, each in 100 μl PBS were incubated for 30 min at 20°C in the presence (5 μg) or absence of sCD4. Each sample was divided into two equal portions. CAP in 0.1 M acetate pH 6.0 was added to one aliquot (final concentration 5 mg/ml). An equivalent amount of acetate without CAP was added to the second aliquot. After incubation for 5 min at 37°C, the suspensions were cooled on ice and PEG was added to a final concentration of 3% to separate HIV-1 from sCD4 and CAP. After 90 min at 4°C, the mixtures were centrifuged at 10,000 rpm, the supernatant fluids removed and the pellets washed once with 3% PEG in PBS containing 10 mg/ml BSA. The final washed pelleted viruses were suspended in PBS containing 100 μg/ml of each BSA and gelatin, and dilutions (indicated on the abscissa) in the same buffer were added to mAb NC-1 coated and control wells. After 5 h at 4°C, the wells were washed and bound virus was quantitated by ELISA for p24 antigen. The amount of HIV-1 IIIB and HIV-1 BaL, respectively, bound to control wells was ≤ 2.1% and ≤ 6.1% of that bound to mAb NC-1 coated wells. All experiments were done at least in triplicate.