GHOST CXCR4 cells (10 4 cells per 96-well) cultured in DMEM/10% FCS were infected with 1000 TCID of the HIV laboratory isolates NL4-3 and were also infected in the presence of 50 μg mHSA per ml cell culture medium (DMEM/10% FCS). Virus and mHSA were added to the cells simultaneously. The cultures were assessed microscopically 5 days after infection by visualization of induced syncytia and cell layer destruction. To visualize syncytia, cell layers were treated with 3% formalin for 10 min to inactivate virus and then fixed with methanol (Solution 1 of the Azur B and Eosin G based Hemacolor ® rapid staining set, Merck, Germany) for 5 min. After fixation, cells were stained with color reagent red (solution 2 of the Hemacolor ® staining set) for 30 sec followed by a staining with color reagent blue (solution 3 of the Hemacolor ® staining set) for 30 sec. Alternatively, HeLa-P4 cells, cultured in DMEM/10% FCS, expressing human CD4, CXCR4 were transfected with 1 μg pSVATGrev plasmid. This plasmid expresses the gp160 protein precursor, which is processed and expressed on the outside of the cellular membrane as the functional gp120/gp41 complex. Cells were transfected for induction of syncytia and mHSA was given to the cells 30 min after transfection. Syncytium formation was assessed after 28 h by standard phase contrast microscopy.