Twenty amino acid residues peptides derived from the N-terminal domain of glycoprotein gp41 exhibit four different types of reactivity with human sera in the course of seroconversion. The differences in the accessibility of the peptide by immune apparatus can result from different binding of the knobs formed by trimers of glycoprotein gp120 to the surface of the viral particle. The region covered with peptides of higher immunoresponse may be involved in binding of knobs to the viral surface. The loss of knobs during ageing is probably the source of differences in the time-dependence of immunoresponse of different epitopes. Peptide fragments covering amino acids 580-588 and 592-612 have been already studied in detail from the point of view of their reactivity with sera and structure. Here we describe new reactive region between the residues 612-629. The structure features of corresponding peptide in solution (studied by 1H NMR spectroscopy) are different from those previously reported. In this particular case peptides corresponding to different regions of gp41 differ also in their structures in solution prior to the binding to the antibody.