Vaccines that elicit broadly neutralizing antibodies to the conserved stem of hemagglutinin (HA) are being developed as universal influenza vaccines that protect against influenza across multiple years. However, different influenza strains, even those in the same subtype with identical stem sequences, can vary in susceptibility to broadly neutralizing stem antibodies, and the reasons are not understood. Here we studied potential mechanisms underlying different sensitivities of a panel of H5N1 HA-pseudoviruses to broadly neutralizing stem antibodies. We found that greater HA conformational stability, as measured by thermal inactivation and pH triggering of conformational changes, correlates with reduced neutralization sensitivity and antibody binding to HA under neutral and low pH conditions. Our data indicate that conformational stability of HA is an important attribute of susceptibility to broadly neutralizing stem antibodies and is influenced by residues outside the stem antibody epitopes.IMPORTANCE The influenza surface glycoprotein hemagglutinin (HA) mediates virus attachment and membrane fusion between virus and host cells, allowing the viral core to enter the host cell cytoplasm for replication. Fusion occurs when HA undergoes low-pH-induced-conformational changes during endocytosis. Broadly neutralizing antibodies to the conserved stem region of HA interfere with conformational changes required for fusion. Vaccines that elicit such antibodies are being developed as novel universal influenza vaccines for multiyear protection. We investigated why H5N1 HA from different strains vary in sensitivity to broadly neutralizing stem antibodies despite having conserved epitopes. We report that HA conformational stability due to residues outside the antibody binding site accounted for much of the variation in susceptibility to neutralization by stem antibodies. These findings highlight the importance of non-epitope residues in influencing neutralization sensitivity to stem antibodies and the complexities in developing universal vaccines targeting conserved epitopes in the HA stem.