One of the hallmarks of Alzheimer pathology is extracellular deposition of beta-amyloid protein (BAP) which is derived from a larger glycoprotein called amyloid precursor protein (APP). Although APP has often been described as a surface membrane protein, such a localization has not previously been demonstrated at the light or electron microscopic level. We now report the results of immunoelectron microscopy using three specific antibodies against different synthetic fragments of APP. All three antibodies demonstrated a major localization to organelles such as the Golgi apparatus, endoplasmic reticulum and vesicular-like structures. A minor proportion of staining with all three was on selective postsynaptic membranes of asymmetrical synapses, whereas staining of presynaptic membranes was not observed. The morphological evidence suggests that one role of APP may be in association with the function of selective synapses.