To investigate the possible relationship between acetylcholinesterase (AChE)-containing fiber density and senile plaque density and between AChE-positive plaques and β/A4 protein deposition, AChE histochemistry, the modified Bielschowsky's method and β/A4 protein immunohistochemistry were performed on the amygdala of Alzheimer's disease (AD) and aged control cases. Abundant AChE-positive senile plaques were found in the amygdala and related structures in AD. These AChE-positive plaques were mainly of the primitive or diffuse type. In addition to senile plaques of typical morphologies a variety of AChE-positive structures were observed in the amygdala and related regions in AD. A comparison of serial sections stained alternatively with AChE histochemistry and β/A4 protein immunohistochemistry has revealed that these AChE-positive structures with variable morphological appearances displayed β/A4 protein immunoreactivity, indicating that AChE is localized in a variety of β/A4 protein deposition including the diffuse plaque. Thus, it is suggested that AChE is present in some senile plaques at the earliest stage. However, there was no apparent correlation between the numerical density of AChE-positive fibers and senile plaque density. These findings suggest that the degeneration of cholinergic neurons is not attributed to the occurrence of AChE activity in β/A4 protein.