Frontal and temporal cortical tissue from the brains of elderly cases of Down's syndrome was used to make preparations of neuronal cell bodies containing senile plaque cores. Polarisation microscopy revealed normal "classical" plaque cores, and also a high proportion of unusual "amorphous" plaque cores which we have not seen in Alzheimer's disease. These two forms were easily distinguished by electron microscopy. This suggests that late Down's syndrome may not be an exact model for Alzheimer's disease.