The region of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 that engages its primary cellular receptor CD4 forms a site of vulnerability to neutralizing antibodies. The monoclonal antibody b12 exploits the conservation and accessibility of the CD4-binding site to neutralize many, though not all, HIV-1 isolates. To understand the basis of viral resistance to b12, we used the atomic-level definition of b12-gp120 contact sites to study a panel of diverse circulating viruses. A combination of sequence analysis, computational modeling, and site-directed mutagenesis was used to determine the influence of amino acid variants on binding and neutralization by b12. We found that several substitutions within the dominant b12 contact surface, called the CD4-binding loop, mediated b12 resistance, and that these substitutions resided just proximal to the known CD4 contact surface. Hence, viruses varied in key b12 contact residues that are proximal to, but not part of, the CD4 contact surface. This explained how viral isolates were able to evade b12 neutralization while maintaining functional binding to CD4. In addition, some viruses were resistant to b12 despite minimal sequence variation at b12 contact sites. Such neutralization resistance usually could be reversed by alterations at residues thought to influence the quaternary configuration of the viral envelope spike. To design immunogens that elicit neutralizing antibodies directed to the CD4-binding site, researchers need to address the antigenic variation within this region of gp120 and the restricted access to the CD4-binding site imposed by the native configuration of the trimeric viral envelope spike.