Fusion inhibition by PRO 542 and AMD 3100. Magi +/+ cells were preincubated for 2 hours at 4°C. Unbound virus was washed away with PBS. Fusion inhibitors AMD 3100 (a CXCR4 antagonist) or PRO 542 (a neutralizing antibody against gp120) were added to cells along with wild type HIV-1NL4.3 at the onset of the 23°C incubation period (white bars) or after 23°C-TAS was established by two hours (black bars). Inhibitors were allowed 1 hour for binding then washed. Following the 3 hour TAS incubation period, the cells were washed with PBS and shifted to 37°C to promote fusion. For control experiments (grey bar), TAS was established, subsequently shifted to 37°C, however no inhibitor was added to block fusion. The averages of triplicate experiments are shown (n = 4). Error bars represent the standard error of the mean. The concentrations of inhibitors were: 4 uM of AMD 3100 and 5 ug/mL of PRO 542.