Binding of HIV-1 and HIV-1-CD4 complexes to CAP coated wells. Suspensions (50 μl) of purified HIV-1 IIIB (6.8 × 109 virus particles/ml) and of HIV-1 BaL (2.5 × 109 virus particles/ml), respectively, in 0.1 M sodium acetate pH 7.0 were mixed with 5 μg of sCD4 and incubated for 30 min at 20°C. sCD4 was not added to control virus preparations. The suspensions were cooled on ice and polyethylene glycol 6000 (PEG) was added to a final concentration of 3% to separate HIV-1 from sCD4 (which does not precipitate in 3% PEG). 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 pellets were resuspended in sodium acetate buffer pH 7.0 containing 25 μg/ml BSA, and serial dilutions (indicated on the abscissa) were added to CAP coated and control wells, respectively. After incubation for 5 h at 4°C, the supernatant fluids were removed, the wells washed, and the bound virus quantitated by ELISA for p24 antigen. The amount of HIV-1 IIIB and HIV-1 BaL bound to control wells was ≤ 2.1% and ≤ 13%, respectively, of that bound to CAP coated wells. All experiments were done at least in triplicate.