Abstract Reagents which block the interaction of HIV-1 gp120 with CD4 + T cell are of therapeutic interest. We assessed the ability of the murine monoclonal antibody (mAb) 87-135/9 to bind to the rgp120 C1 region and to block the CD4 interaction, and compared this with the reactivity of the rat mAb 388/389 or the human mAbs F105 and b12, which are known to bind within or near the gp120 C4 region. ELISA and surface plasmon resonance measurements showed that mAb 87-135/9 recognized specifically the gp120 C1 peptide HEDIISLWDQSLK (residues 105–117). All four mAbs bound to rgp120 and blocked its interaction with CD4 + T cells. When mAb 87-135/9 was used in combination with one of the other antibodies, its inhibitory effect was additive. A therapeutic approach could be to use a human anti-gp120/CD4bs conformational mAb in combination with a humanized Ab directed against the conserved, linear gp120 C1 epitope and/or an anti-viral drug to hinder the HIV-1 virus and shedded envelope protein to bind to CD4 + T cells.