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Aims: Archived tissue blocks preserve the antigenicity of samples for a long time under normal storage conditions, whereas tissue sections may show a diminished immunoreactivity over time. Little is known about the processes responsible for antigenicity loss and how tissue sections should be conserved for extended storage. Oxidation and drying are presumed mechanisms of antigenicity loss. To prove this, degradation of immunoreactivity was provoked by chemical oxidation, photo-oxidation and artificial drying., Methods: First, paraffin sections of an oestrogen receptor (ER) positive breast carcinoma were subjected for variable time periods to H2O2, ultraviolet A (UVA) irradiation and dry heat (56 degrees C) prior to ER immunohistochemistry. Second, using heat and UVA irradiation several other antigens (ER, PR, HER-2neu, p53, p63, p16, PSA, CK5/6, CK7, CK20, SMA, Fli-1, c-kit, CD20 and EGFR) were tested, using internal control tissue microarray sections., Results: While H2O2 had no effect on the ER-staining intensity, extended drying showed a detectable decrease in staining after 10 days, and UVA irradiation induced a decrease after 3 days. Entire antigenicity loss was not observed. Except for HER-2neu, PSA and Fli-1, all antigens showed some diminution in antigenicity, but no entire antigenicity loss, after heating and UVA irradiation., Conclusions: This study confirms that photo-oxidation and drying have an influence on immunohistochemistry outcome, and protocols for testing the stability of specific antigens are provided.