Human PrP (residues 91-231) expressed in Escherichia coli can adopt several conformations in solution depending on pH, redox conditions and denaturant concentration. Oxidised PrP at neutral pH, with the disulphide bond intact, is a soluble monomer which contains 47% alpha-helix and corresponds to PrPC. Denaturation studies show that this structure has a relatively small, solvent-excluded core and unfolds to an unstructured state in a single, co-operative transition with a DeltaG for folding of -5.6 kcal mol-1. The unfolding behaviour is sensitive to pH and at 4.0 or below the molecule unfolds via a stable folding intermediate. This equilibrium intermediate has a reduced helical content and aggregates over several hours. When the disulphide bond is reduced the protein adopts different conformations depending upon pH. At neutral pH or above, the reduced protein has an alpha-helical fold, which is identical to that observed for the oxidised protein. At pH 4 or below, the conformation rearranges to a fold that contains a high proportion of beta-sheet structure. In the reduced state the alpha- and beta-forms are slowly inter-convertible whereas when oxidised the protein can only adopt an alpha-conformation in free solution. The data we present here shows that the human prion protein can exist in multiple conformations some of which are known to be capable of forming fibrils. The precise conformation that human PrP adopts and the pathways for unfolding are dependent upon solvent conditions. The conditions we examined are within the range that a protein may encounter in sub-cellular compartments and may have implications for the mechanism of conversion of PrPC to PrPSc in vivo. Since the conversion of PrPC to PrPSc is accompanied by a switch in secondary structure from alpha to beta, this system provides a useful model for studying major structural rearrangements in the prion protein.