Synergism between CAP and sCD4 for virucidal activity against HIV-1 IIIB. sCD4 was added to 50 μl of HIV-1 IIIB (6.8 × 109 virus particles/ml) in PBS. After 30 min at 20°C, CAP was added to final concentrations between 0 and 1250 μg/ml and the mixtures were incubated for 5 min at 37°C. CAP at the same concentrations described above was also added to virus not pretreated with sCD4. Virus suspensions were put on ice and PEG was added to a final concentration of 3% to separate HIV-1 from CAP and sCD4. After 90 min at 4°C, the mixtures were centrifuged at 10,000 rpm, the supernatant fluids removed and the pellets washed twice with 3% PEG in PBS containing 10 mg/ml BSA. The final washed pellets were resuspended in tissue culture medium, and serial 2-fold dilutions in the same medium were added to MT-2 cells. Infection was monitored by ELISA for p24 antigen. The gray horizontal line corresponds to virus inactivation (80.2 ± 1.3% of residual infectivity) caused by treatment with sCD4 in the absence of CAP. All experiments were done at least in triplicate.