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The transmembrane (T) domain of diphtheria toxin has a critical role in the low pH–induced translocation of the catalytic domain (A chain) of the toxin across membranes. Here it is shown that at low pH, addition of proteins in a partly unfolded, molten globule-like conformation converted the T domain from a shallow membrane-inserted form to its transmembrane form. Fluorescence energy transfer demonstrated that molten globule-like proteins bound to the T domain. Thus, the T domain recognizes proteins that are partly unfolded and may function in translocation of the A chain as a transmembrane chaperone.