To determine the correlation between the immunoreaction against the core structure of human immunodeficiency virus type(HIV-1)transmembrane protein gp41 epitopes and the disease progression,it is essential to evaluate the anti-core structure antibody epitopes and the humoral immunity against the epitopes.For this purpose we evaluated monoclonal antibodies(mAbs)against the gp41 core structure such as mAbs 50.69,98.6 and T26,byWestern blotting(WB)and flow cytometry.WB showed mAbs 50.69 and 98.6 bound to both monomeric and oligomeric gp41,and mAb T26 exclusively bound to oligomeric gp41.We evaluated the sera from Pneumocystis pneumonia patients(PCP;n=7)and long-term survivors (LTS; n=7). Competition assay with sera and mAbs for binding to H9 cells infected with HIV-1 IIIB virus was done using flow cytometry.The results revealed that PCP sera as well as LTS sera inhibited the binding of all the three mAbs,and the PCP sera inhibited mAb T26 binding more efficiently than LTS.Therefore,PCP patients retain competing immunity to antibodies against not only the shared epitopes of the core structure(binding sites of mAbs 50.69 and 98.6)but also against oligomeric gp41 specific epitope(binding site of mAb T26).