The Respiratory Syncytial Virus (RSV) fusogenic glycoprotein F₁ was characterized using biochemical and biophysical techniques. Two heptad-repeat (HR) regions within F₁ were shown to interact. Proteinase-K digestion experiments highlight the HR1 region (located proximal to the fusion peptide sequence) of the F₁ protein to which an HR2-derived (located proximal to the membrane-spanning domain) peptide binds, thus protecting both the protein and peptide from digestion. Solution-phase analysis of HR1-derived peptides shows that these peptides adopt helical secondary structure as measured by circular dichroism. Sedimentation equilibrium studies indicate that these HR1 peptides self-associate in a monomer/trimer equilibrium with an association constant of 5.2 × 10⁸ M^-2. In contrast, HR2-derived peptides form random monomers in solution. CD analysis of mixtures containing peptides from the two regions demonstrate their propensity to interact and form a very stable (T_m = 87 °C), helical (86% helicity) complex comprised of three HR1 and three HR2 members.