Since apolipoprotein E immunoreactivity has been shown in senile plaque and neurofibrillary tangle in Alzheimer's disease brains, we performed immunohistochemistry to examine whether or not apolipoprotein B (apoB) was associated with these abnormal structures. Sections of hippocampus from formalin-fixed, paraffin-embedded tissues were immunostained with a standard ABC method. We found that apoB immunoreactivity was associated with amyloid both in senile plaques and cerebral vessels and with neurofibrillary tangles. Positive staining was abolished with pretreatment of the antibody with the human apoB. It is suggested that the potential blood-brain barrier impairment may allow an increasing amount of serum proteins, such as apoB, to penetrate the brain and bind to these AD-associated abnormal structures.