For immunohistochemistry of the prion diseases, several pretreatment methods to enhance the immunoreactivity of human and animal abnormal proteinase-resistant prion protein (PrPSc) on the tissue sections have been employed. The method of 121 degrees C hydrated autoclaving pretreatment or the combination method of 121 degrees C hydrated autoclaving with a certain chemical reagent ( formic acid or proteinase K, etc) are now widely used. We found that an improved hydrated autoclaving method at 135 degrees C, more effectively enhanced PrPSc immunoreactivity for the antibodies recognizing the linear epitope. In addition, this method was more effective for the long-term fixation samples as compared with other previous methods. However, this modified method could not retrieve PrPSc antigenic epitopes composed of conformational structures or several discontinuous epitopes. We describe the comparative studies between our improved method and other antigen-retrieval procedures reported previously. Based on the differences of reaction among the antibodies, we also discuss the mechanisms of the hydrated autoclaving methods to retrieve PrPSc immunoreactivity.