PROTEIN folding in vivo is mediated by helper proteins, the molecular chaperones(1-3), of which Hsp60 and its Escherichia coli variant GroEL are some of the best characterized. GroEL is an oligomeric protein with 14 subunits each of M(r) 60K(4-6), which possesses weak, co-operative ATPase activity(7-9) and high plasticity(10). GroEL seems to interact with non-native proteins, binding one or two molecules per 14-mer(11-19) in a ’central cavity’(20), but little is known about the conformational state of the bound polypeptides. Here we use nuclear magnetic resonance techniques to show that the interaction of the small protein cyclophilin(21,22) with GroEL is reversible by temperature changes, and all amide protons in GroEL-bound cyclophilin are exchanged with the solvent, although this exchange does not occur in free cyclophilin. The complete secondary structure of cyclophilin must be disrupted when bound to GroEL.