The human immunodeficiency virus (HIV) inhibitor AR177 (T30177, Zintevir) has been identified as a potent inhibitor of HIV integrase in vitro. The compound is currently the subject of clinical phase I/II trials. However, the primary target for the mechanism of action in vivo has not been identified unequivocally. We have found that AR177 inhibits syncytium formation between MOLT-4 cells and HUT-78 cells persistently infected with the HIV-1IIIB or NL4-3 strain, at a 50% effective concentration of 3 microg/ml, roughly 3-fold higher than the concentration required to inhibit HIV replication. Furthermore, flow cytometric analysis has shown that AR177 at 25 microg/ml interferes with the binding of the monoclonal antibody 9284 (directed to the V3 loop of gp120) on HIVIIIB-infected HUT-78 cells, pointing to inhibition of virus binding or virus fusion as the mechanism of action of AR177. To precisely characterize the site/target of intervention by AR177, we have selected HIV-1 (NL4-3) strains resistant to AR177. The binding of the AR177-resistant strain, unlike the parental HIV-1 NL4-3 strain, could not be inhibited by AR177. The resistant phenotype was associated with the emergence of mutations in the gp120 molecule. DNA sequence analysis revealed the presence of the K148E, Q278H, K290Q, and F391I mutations and a deletion of 5 amino acids (FNSTW) at positions 364-368 in the V4 region of the resistant strain but not of the wild-type HIV strain. Selection of resistant strains, although it takes a relatively long time to develop, may also select for strains with lower replicative capacity. No mutations were found in the integrase enzyme gene. Our data argue against HIV integrase being the primary target for the mechanism of anti-HIV action of AR177.