Amino acid sequences of the HIV-1 surface protein gp120 show extensive variations. Some of the peptide or DNA vaccines using the V3-loop of gp120 can trigger antibody production against some isolates of HIV, but these antibodies are not protective against other isolates. Thus, more conserved peptides from other HIV proteins have also been tried as vaccines. However, antibodies specific for these peptides cannot prevent the normal entry of HIV into the host cells, followed by integration of viral genomes into host chromosomes. Therefore, we decided to make a careful analysis of the aligned amino acid sequences of gp120 stored at Los Alamos National Laboratory. Unexpectedly, we found that there are several, not just one or two, segments of seven or more consecutive and nearly invariant amino acid residues present in the C1, C2 and C5 regions of this protein. Furthermore, four of these segments are physically close to each other in the known three-dimensional structure of the core portion of gp120. The remaining segments are not in that crystallized region; however, two of these are covalently linked with a disulfide bond. Therefore, we propose that these segments may be combined to form the basis of possible HIV vaccine candidates. In addition, it is necessary to incorporate the processed peptide from gp120 or other proteins and to understand the time-dependent effects of the highly variable loops of gp120 on triggering antibody production in human patients.