Activity of CCR5 ECL peptides on Envs from primary HIV-1 isolates. Effector cells (HeLa cells transfected with plasmids containing the designated primary Envs linked to the T7 promoter and infected with vTF7-3) were preincubated for 1 hour at 37°C with the P3 peptide at the indicated concentrations. Target cells were prepared by coinfecting NIH-3T3.T4 cells with the vaccinia recombinants encoding CCR5 (vHC-1) plus vCB21R-LacZ. Fusion assay was performed as described in “Materials and methods.” β-Galactosidase values (10^–3 OD/min) were: Ba-L, 9.5; 92UG024.2, 7.5; 92HT93.1, 9.0. Error bars indicate SD of mean values obtained from duplicate fusion assays.