The purple membrane of Halobacterium salinarium is a two-dimensional lattice of lipids and the integral membrane protein bacteriorhodopsin (BR). To determine whether helix-helix interactions within the membrane core stabilize this complex, we substituted amino acid residues at the helix-helix interface between BR monomers and examined the assembly of the protein into the lattice. Lattice assembly was demonstrated to fit a cooperative self-assembly model that exhibits a critical concentration in vivo. Using this model as the basis for a quantitative assay of lattice stability, bulky substitutions at the helix-helix interface between BR monomers within the membrane core were shown to be destabilizing, probably due to steric clash. Ala substitutions of two residues at the helix-helix interface also reduced stability, suggesting that the side chains of these residues participate in favorable van der Waals packing interactions. However, the stabilizing interactions were restricted to a small region of the interface, and most of the substitutions had little effect. Thus, the contribution of helix-helix interactions to lattice stability appears limited, and favorable interactions between other regions of neighboring BR monomers or between BR and lipid molecules must also contribute.