Immunolabelling was performed on brain sections of scrapie-infected mice with antibodies against tau, a microtubule-associated protein, and against the paired helical filaments of Alzheimer's disease. Both kinds of antibodies have been shown previously to label paired helical filaments in neurons and in abnormal neurites associated with the senile plaques of Alzheimer's disease. Amyloid plaques in murine scrapie were labelled by both antisera. The structural substrate for the immunolabelling was probably the dystrophic neurites in the periphery of the plaques, leaving the amyloid core unstained. These results emphasize that similar mechanisms are likely to be involved in the pathogenesis of plaques in both diseases.