It has been shown in previous reports that a guanine-rich phosphodiester oligonucleotide bearing a dimethoxytrityl (DmTr) residue on its 5'-terminal. DmTr-TGGGAGGTGGGTCTG (SA-1042), is an inhibitor of HIV-1 infection in vitro. SA-1042 interfered with the attachment of gp120 to the CD4 receptor and the subsequent entry stage of viral infection. We investigated the structure-activity relationship of the DmTr-conjugated oligomer by using 15-mer oligonucleotides with various nucleotide sequences. Results show that location of guanine nucleosides at the 5'-terminal and modification of the 5'-terminal with DmTr are essential for anti-HIV-1 activity. First, substitution of the guanine nucleoside close to the 5'-terminal of SA-1042 with another nucleotide prevented antiviral activity. Second, the existence of at least three consecutive guanine nucleosides adjacent to the 5'-terminal was required for the activity. Finally, modification of the 5'-terminal was essential for the activity. Based on these findings, the hexanucleotide, DmTr-TGGGAG, was identified as a potent inhibitor of HIV-1 infection. The hexamer was found to be capable of inhibiting the binding of gp120 to its receptor CD4 molecule, and it was also capable of inhibiting accessibility of anti-V3 monoclonal antibody to its ligand V3 peptide.