Loss of antigenicity or epitope instability, especially after tissue sections have been prepared, may affect immunohistochemical staining results. To obtain insight into the magnitude of this problem, we conducted a systematic investigation. A total of 20 monoclonal and 12 polyclonal antibodies were used for immunohistochemistry using formalin-fixed, paraffin-embedded sections that were stored for periods ranging from 15 to 360 days at temperatures of 4 degrees C, 21 degrees C, and 37 degrees C. After 1 year of storage. immunohistochemical results appeared to be impaired with four antibodies at 4 degrees C, 11 antibodies at 21 degrees, and 16 antibodies at 37 degrees C. It was found that staining results using polyclonal antisera are affected to the same extent by this phenomenon compared to antibodies of monoclonal origin. No indication was found that antigen retrieval could restore the effect of epitope instability. An additional experiment, in which the influence of section adhesives on epitope instability was investigated, revealed that none of the applied adhesives hampers immunohistochemical reactivity for a storage time of up to 6 months.