Binding of C-18 to CCR5 is partially blocked by MIP-1β and R5 (BaL) envelope. CEM.NKR-CCR5 cells were incubated with increasing concentrations of unlabeled C-18, human MIP-1β, R5 (Ba-L) Env, or X4 (MN) Env prior to addition of a fixed concentration of ¹²⁵I-C-18. After incubation for 90 minutes at 4°C, the bound counts were measured as described in “Materials and methods.”