HIV-1 strain NL4-3, an X4-monotropic laboratory isolate, was used for virus inhibition tests. HeLa-P4 cells expressing human CD4, CXCR4 were kindly provided by Matthias T. Dittmar, University Heidelberg. GHOST-CXCR4 cells, antibody sera, monoclonal antibody, and gp41 (ARP671, spanning the entire extracellular domain of HIV-1 gp41 MN ) were obtained from the European Vaccine Against AIDS Programme (EVA, Potters Bar, UK). Recombinant HIV-1 gp120 (LAV, CXCR4 monotropic, identical to NL4-3 gp120, glycosylated) and human rCD4 (soluble form, transmembrane region removed, glycosylated) was obtained from Protein Sciences Corporation, Meriden, CT, USA. These commercially proteins were expressed in insect cells by a baculovirus vector system, secreted in the serum-free cell culture medium and purified under non-denaturing conditions. The env expression plasmid pSVATGrev carries a subgenomic HIV NL4-3 fragment coding for rev, vpu and env under the transcriptional control of the SV40 early promotor. The plasmid sequence can be obtained upon request. Construction of NL4-3 mutants with R5X4 and R5 tropism and mutants lacking N-glycosylation site g15 was carried out as described earlier [17] .