One of the limitations of immunoperoxidase methods applied in surgical pathology is the loss of immunoreactivity during fixation and processing. In this study, the effects of decalcification on the immunoreactivity of various antigens in formalin fixed tissues were examined. The pieces of fixed tissue were placed in four different decalcifying solutions (EDTA, formic acid, nitric acid, and Plank-Rychlo solution) for various time periods and then processed routinely and embedded in paraffin. The paraffin sections of the tissues were stained with immunoperoxidase methods for immunoglobulins, hormones, or tissue/tumor-specific markers. Sections of nondecalcified tissues were used as a control. The immunohistochemical staining intensity and the number of positive cells were not reduced significantly in decalcified tissues within the usual time period of routine decalcification procedures. Many antigens survived even prolonged decalcification. The results of this study indicate that routinely decalcified tissues can be used for immunoperoxidase staining without significant loss of immunoreactivity.