To determine whether the amino acid sequence extending from residue 273 to residue 288 in the second conserved region C2 of the HIV-1 envelope glycoprotein represents a target for antibodies on monomeric and oligomerized HIV-1 gp120env, we characterized several antisera and monoclonal antibodies (MAb) raised against C2 synthetic peptides. A cross-reactive epitope was evidenced on HIV-1Lai and HIV-1Eli C2-derived peptides, but was not encountered on HIV-2 C2-derived peptide. This epitope was found to be expressed on the native monomeric gp120env but was not detected in the context of oligomeric Env, suggesting this region is sequestered in the oligomeric molecule. Preincubation of oligomeric Env with sCD4 apparently failed to expose this epitope. Our results suggest that the amino acid sequence extending from residue 273 to residue 288 in C2 of HIV-1 gp120env may be involved in intermolecular interaction within the oligomeric Env complex.