Specific Inhibition of HIV-1 Env-mediated cell fusion by the CCR5-derived peptides. NIH-3T3 cells expressing vaccinia-encoded CD4 (vCB-3), transfected CCR5, and T7 RNA polymerase (vTF7-3) were used as target cells. Peptides were added to the target cells at the indicated concentrations and incubated for 1 hour at 37°C. For effector cells, HeLa cells were infected with the vaccinia recombinant containing the T7 promoter-lacZ reporter gene (vCB-21R-lacZ) and infected with vaccinia virus encoding Ba-L (vCB-43). For 89.6 Env expression on HeLa cells, the cells were transfected with pSC59/89.6 and then infected with vCB21R-lacZ. The fusion assay was performed as described in “Materials and methods.” β-Galactosidase values (10^–3 OD/min) were: Unc, 0.5; Ba-L, 8.5; 89.6 Env, 7.5. Error bars indicate SD of mean values obtained from duplicate fusion assays.