Kuru, Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler syndrome (GSS) are transmissible dementias affecting humans characterized neuropathologically by intraneuronal vacuolation, spongiform change, astrocytic hypertrophy and hyperplasia and the variable presence of amyloid plaques. It has been suggested that microglia are amyloid-forming cells, which play an essential role in amyloid plaque formation. To study the relationship between microglia and amyloid plaques in kuru, CJD and GSS, cerebellar tissues were examined by the double-immunostaining technique using anti-ferritin antibodies as the microglial marker and anti-scrapie amyloid antibody as plaque marker. Ferritin-immunoreactive microglia were observed interdigitating with and among unicentric, multicentric and diffuse types of scrapie amyloid-immunoreactive plaques and were found to a lesser extent in the neuropil. In kuru and CJD, scrapie amyloid-immunoreactive plaques were predominantly unicentric and were observed in the granular layer. In kuru, 53% of the amyloid plaques were associated with microglia, whereas only 30% of plaques in CJD were. In contrast, scrapie-amyloid-immunoreactive plaques in GSS were of the multicentric type, predominantly observed in the molecular layer, and 90% of these plaques were associated with microglia. Our data indicate that microglia are frequently associated with scrapie amyloid-immunoreactive plaques in GSS, less commonly in kuru and to a much lesser extent in CJD, suggesting that microglia may play a variable but important role in the formation of plaques in the transmissible spongiform encephalopathies.