Structural changes of native and regenerated silk fibroin membranes were induced by immersion in water-methanol solutions and examined as a function of immersion time and methanol concentration. X-ray diffractometry and infrared spectroscopy data showed that transition from random coil to beta-sheet structure occurred favorably when both native and regenerated silk fibroin membranes were immersed in water-methanol solutions, regardless of the different immersion time. Only native silk membrane, treated for 2 min with pure methanol, maintained its original amorphous structure, as demonstrated by differential scanning calorimetric (DSC) curves. The degree of displacement, measured by thermomechanical analysis (TMA), was much greater for regenerated than for native silk fibroin membranes. SDS-PAGE pattern showed that native silk fibroin has a molecular weight of 350, while the regenerated sample is formed by a large number of polypeptides in the range of 200-50 KD. The amount of acidic and basic amino acids decreased slightly in regenerated silk fibroin. Physical properties of silk membranes treated with water-methanol solutions are discussed in terms of membrane structure, treatment conditions, and chemical structure of starting material.