Comparison of the effects of modulating Env versus coreceptor expression levels on the efficiency of peptide blocking. Effector cells were prepared by infecting HeLa cells with vCB21R-LacZ and expressing the Ba-L Env either by plasmid transfection (Low Ba-L) or recombinant vaccinia virus infection (Hi Ba-L). Target cells were prepared by infecting HeLa.T4 cells with vTF7-3 and expressing CCR5 either by plasmid transfection (Low CCR5) or recombinant vaccinia virus infection (Hi CCR5). The P3 peptide was preincubated with the effector cells, and RANTES was preincubated with the target cells for 45 minutes at 37°C. Effector and target cells were mixed in the indicated combinations, and cell fusion was measured as described in “Materials and methods.” Error bars indicate SD of mean values obtained from duplicate fusion assays.