A phage display library screening approach was used to identify peptide sequences that could bind to anti-HIV-1 MAbs whose binding specificities are complex. Most of the antibodies used recognize discontinuous epitopes in gp120 and one recognizes gp41. Both a 15-mer and a 21-mer display library (each with a complexity of greater than 60 x 10[6]) and two constrained, V3 region-biased libraries, all expressed as recombinant pIII protein of filamentous phage, were used. The unmapped anti-gp120 human MAb A32 recognized a set of related linear sequences and repeatedly identified a single phage sequence that could form a cyclic disulfide structure. Selection methods were also developed so that phage could be obtained by competition selection in the presence of antibody bound to native, monomeric gp120 antigen (used with MAb IgG1b12 and the anti-gp120 V3 region MAb 447-52D) or gp120 variable region 3 synthetic peptides (used with anti-gp120 V3 region MAb 19b). The potent, virus-neutralizing MAb IgG1b12 recognized numerous sequences and, when used in competition with gp120, recognized only one sequence. These studies extend the range of antibody determinant studies that can be performed with display phage libraries, demonstrate a workable experimental strategy for use of competition ligands to discriminate among phage mimotopes, and provide a large number of mimotopes that bind potent virus-neutralizing MAbs for HIV-1 vaccine studies.