Annexin B12 (ANX) is known to insert across the lipid bilayer at acidic pH in the absence of Ca²⁺ and to form a pore-like structure consisting of several transmembrane helices, most of which are unknown. Our previous studies demonstrate that the insertion proceeds via an interfacial refolded intermediate state, which can be stabilized by anionic lipids. Energy transfer measurements in a mixture of donor- and acceptor-labeled ANX indicate that this interfacial intermediate, unlike the final transmembrane conformation, is prone to aggregation. Such aggregation of a non-inserted ANX may have implications for a possible general mechanism of misfolding of membrane proteins.