HIV-1 gp120 competition assay. Both CEMss and UT-7 cells were labeled with (I) PE-conjugated unspecific mouse IgG; (II) Leu3A anti-CD4 MoAb; and (III) Leu3A MoAb after pretreatment with saturating amounts of HIV-1 recombinant gp120 protein. M1, marker 1.