It has been observed that immunoreactivity in paraffin sections decreased during storage. In this study, stored paraffin sections from both biopsy material and cultured cells were assessed for changes in immunoreactivity, using color-based image analysis to quantitate extent and intensity of the stainings. For seven of the 11 antibodies studied, storage at 20 degrees C for 16 weeks reduced the extent of immunostaining compared with that of freshly cut sections. Furthermore, increased storage temperatures resulted in a progressive loss of immunoreactivity. After 2 weeks of storage, at both 4 degrees C and 20 degrees C, p53 protein- and MIB1-antigen expression was significantly reduced regarding extent and intensity. The extent of the immunoreactivity reduced more for p53 protein than for MIB1 antigen, but the intensity did not. Boric acid was used for antigen retrieval on sections stored for 12 weeks at 20 degrees C. For both p53 protein and MIB1 antigen, this resulted in an extent and intensity of immunostaining equal to or higher than (MIB1) that obtained in freshly cut sections, using citrate buffer. Staining of cultured cells confirmed the results from biopsy material on the influence of storage temperature. Fixation time only marginally influenced the storage-related decrease in immunoreactivity. In conclusion, storage of paraffin sections leads to a varying degree of decreased immunoreactivity for several antibodies. The degree is at least partly dependent on storage time and temperature but not fixation time. However, this may be compensated for by optimizing the antigen retrieval protocol.