To search for brain molecules that share epitopes with HIV (human immunodeficiency virus) proteins, we investigated non-AIDS human brains by immunohistochemical and Western blotting methods, and detected a 23 kDa protein which was immunoreactive with antibodies against HIVp17, a core protein of HIV with a molecular weight of 17 kDa. The 23 kDa protein was localized mostly in reactive astrocytes in and around pathologic areas of the brain, but some neurons also had it. We propose that any HIV vaccine must be checked up if it induces a cross-reacting antibody to a human brain or other organs.