Immunoperoxidase techniques were used to obtain information about the possible presence of serum factors in senile plaques. We found only in plaques consisting of an amyloid core surrounded by a corona of degenerating neurites small amounts of IgG and light chains (kappa and labda). These immunoglobulins were principally localized in the corona and not in the central amyloid core. further it was found that all plaques contain the complement factors C₁q, C₃b, C₃c, C₃d and C₄. Senile plaques lacked C₅, C₃ pro-activator and properdin. The possible significance of these findings in the gensis of the senile plaques and amyloid formation is discussed.