In this study, we have systematically assessed the influence of postmortem delay (PMD) and fixation time (FT) on the immunohistochemical (IHC) staining outcome. The IHC method is frequently applied on surgical and postmortem samples in diagnostics and research. To replicate the routine situation, brain tissues from pigs were exposed to either storage in a refrigerator (+8°C), that is, PMD (1 to 168 h), or fixed in 10% buffered formalin, that is, FT (18 to 94 d). Subsequently, the tissue was routinely processed into paraffin blocks to enable construction of tissue microarrays (TMA). Sections cut from the TMA blocks were stained applying 13 different antibodies directed against neuronal and glial antigens. Immunoreactivity applying 5 antibodies was influenced by prolonged PMD and applying 2 antibodies by prolonged FT. None of the staining outcomes related to the PMD or FT were predictable. Loss of TMA cores during processing was primarily influenced by pretreatment and by tissue characteristics (gray/white matter). The test model described here confirmed that these 2 variables, PMD and FT, indeed influence the IHC outcome. The PMD and FT are particularly of importance while assessing tissue samples obtained at autopsy. The result above is also of importance while comparing the IHC outcomes seen in the postmortem setting (various PMD/FT) with surgical samples or with IHC outcome seen in experimental animal setting (controlled PMD/FT). Thus, we suggest that the test model described here is considered when assessing the reliability of the IHC outcome when analyzing tissues with various characteristics.