In this study, we tested the hypothesis that donors with broadly cross-reactive HIV-1 neutralizing (BCN) sera are infected with viruses encoding envelope glycoproteins (Envs) with unusual immunogenic properties. Cloned env genes were from samples of donors previously identified as having BCN antibodies (BCN donors) and from other donors not known to have such antibodies (non-BCN donors). Neutralization properties of viruses pseudotyped with BCN and non-BCN Envs were determined using BCN, non-BCN sera and broadly cross-neutralizing monoclonal antibodies (Mabs). BCN sera neutralized with higher frequency and geometric mean titers than non-BCN sera. Viruses pseudotyped with BCN Envs were mostly resistant to neutralization by anti-gp120 Mabs but tended to be more sensitive to the anti-gp41 Mabs, 2F5 and 4E10 than non-BCN Env-pseudotyped viruses. Sequence analysis of clones obtained from sequential samples of two BCN donors revealed respective 2F5 epitope mutations T662A and K665T. The K665T mutation evolved as the predominant genotype in the respective donor, consistent with an escape mutation event. The A662T mutation reduced sensitivity to 4E10, as well as 2F5 and homologous sera, consistent with neutralization escape mutation and targeting of the 2F5 epitope region by the serum. Our study suggests that viruses infecting these BCN donors encoded Envs that may have been unusually competent for induction of antibodies against the membrane proximal epitope region (MPER) of gp41, and these Envs may be useful vaccine components.