(a) GHOST-CXCR4 cells were infected with 1000 TCID (Tissue Culture Infective Dose) of the X4-tropic laboratory strain NL4-3. To study inhibition, virus was given to the cells after preincubation of cells with modified serum albumin for 30 min. Cell culture supernatants were tested on day 5 for p24-antigen by a standard p24-antigen ELISA. GHOST cells were cultured in DMEM (Gibco) including 10% FCS (Biochrom KG). (b) TZM-bl cells were infected with virus supernatant representing an infectious dose of 500 foci/96-well. Viral entry into TZM bl cells causes the expression of β-galactosidase, which can be monitored 2 days after infection. To detect βgalactosidase-expressing cells, the cell culture supernatant was removed and the cell layer was treated with 0.25% glutaraldehyde in phosphate-buffered saline (PBS, 5 min, RT) and washed three times with PBS. To stain infected cells, 100 μl of the X-gal solution (0.5 mg X-gal [5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside]/ ml, 1 mM MgCl 2 , 3 mM potassium ferrocyanide, 3 mM potassium ferricyanide in PBS) was added to each well and the plates were incubated at 37°C. The staining reac-tion was monitored by microscopy. Finally, the cell layer was washed again with PBS and blue foci were counted.