The human immunodeficiency virus (HIV) proteins gp120 and gp41 are the principal immune target in HIV infection. One of the most important trends in the study of AIDS is linked to the mapping of sites involving in the binding to the cell receptor CD4 and in the induction of virus-neutralizing antibodies (VNA). Recent studies have revealed that gp120 as the major domain contains inducing type-specific BNA (PND) and a binding region with CD4 (CD4-BR). PND is located in the hypervariable loop of gp120 (residues 301-336 for a BRU strain), and CD4-BR is in the conservation area (residues 410-450). By using the synthetic fragments from these areas (BRU and MN strains) and HIV-infected persons' sera, the authors established that the immune response to PND and CD4-BR is somewhat interrelated: there is a synchronized response of HIV antibodies to peptides from the two regions in ELISA (r = 0.82). For analysis of this phenomenon, experiments with cross-linked immunoreactivity of rabbit antisera to peptides from PND and CD4-BR with homologous and heterologous peptides were performed by applying three control peptides from HIV and hepatitis B virus. It has been found that there is a cross reactivity between rabbit anti-PND (MN, BRU) and anti-CD4-BR abs. Peptide homological analysis revealed common structural elements for PND and CD4-BR despite significant differences in their proposed functions. There is a large amount of positively charged aa within both PND and CD4-BR which may be involved in gp120-CD4 interaction. Acetylation of Lys residues resulted in complete loss of peptide reactivity.