AIMS/BACKGROUND: Intranuclear inclusions have been observed in gestational endometrial glands. Although resembling Herpes simplex virus infected cells, these nuclei have been shown to contain endogenous biotin. Hence, immunohistochemical systems using the avidin-biotin complex will result in cross reaction and false positivity. This study investigates a series of 10 gestational endometria (formalin fixed and paraffin wax embedded) with intranuclear inclusions using three different immunodetection systems with primary anti-biotin. RESULTS: Both the alkaline phosphatase anti-alkaline phosphatase (APAAP) and peroxidase anti-peroxidase (PAP) systems confirmed that the intranuclear inclusions were endogenous biotin. The streptavidin biotin complex (StreptABC) system produced a positive reaction in both test sections and controls (omitting primary anti-biotin) in the presence of prior blocking with free avidin and biotin. In addition, aberrant immunoreactivity was observed in adjacent nuclei/cytoplasm of endometrial glands and decidualised stroma in the negative control of the PAP system. This was subsequently eliminated using microwave pretreatment prior to immunohistochemistry. CONCLUSION: The use of either the APAAP or PAP immunodetection systems (the latter with microwave pretreatment) is recommended for any immunohistochemical or non-isotopic in situ hybridisation investigation undertaken on gestational endometria.