Binding of biotinyl-gp120, biotinyl-sCD4 and biotinyl-gp120-sCD4 complexes to CAP coated wells. Graded quantities of biotinyl-gp120 (indicated on the abscissa) and of biotinyl-gp120-sCD4 complexes were added to CAP coated and control wells. The complexes were prepared by mixing 1 μg of biotinyl-gp120 with 500 ng of sCD4 for 30 min at 20°C in PBS. To determine the effect of sCD4/gp120 ratios on gp120 binding to CAP, constant amounts (500 ng) of biotinyl-gp120 were mixed with graded quantities of sCD4 (0 to 1,000 ng) (insert) and the mixtures were further handled as described above. Binding of biotinyl-gp120 did not increase at sCD4/biotinyl-gp120 weight ratios >1 (data not shown). In control experiments, graded quantities of biotinyl-sCD4 in the absence of gp120 were added to the wells. After incubation at 4°C overnight, the wells were washed and bound biotinylated proteins were detected from subsequent binding of HRP-streptavidin. Absorbance corresponding to biotinylated proteins bound to control wells was in the range of 0–0.026 and was substracted from the absorbance corresponding to biotinylated proteins bound to CAP coated wells.