PM1 cells were infected with the R5 virus (BA-L, JR-CSF) (100 TCID₅₀ per well). PBMCs were stimulated with PHA-P (0.25 μg/mL) and IL-2 (20 U/mL) for 3 days and were then infected with primary isolates (100 TCID₅₀ per well). C-18 protein (at serial 3-fold dilutions) was preincubated with the cells for one hour at 37°C before addition of the virus (5 replicate per group). The virus was washed away after 2 days. Supematants were collected every 2 days and assayed for p24 activity. The 50% viral inhibition dose was calculated according to Reed and Muench as described by Shibata et al. ¹⁶  No inhibition was observed with human cyclophilin.